In published, broadcast, and leaked sources, John Lennon was never asked about the song "Wonderful Christmastime," nor did he offer his opinion.
Between the end of 1975 and the fall of 1980, John Lennon took an extended sabbatical from public life, and there are only about three or so very brief interviews with him during this period. Among them is this one from 1977 where he was cornered by reporter Joel Siegel while he and his wife were taking their son to the circus at Madison Square Garden. Funnily enough, Mick Jagger was with him and ducked out as soon as the reporter started asking questions.
The only one during this period that post-dated Paul's recording and release of "Wonderful Christmastime" (in August and December 1979, respectively) was for a Japanese radio station interviewing his wife Yoko Ono. This was recorded just after New Year's of 1980. He got on the phone for a few minutes, but all the questions were related to his recent visits to Japan with his wife and son Sean. Nothing was asked about Paul McCartney, nor even his own music, before Lennon handed the phone back to Ono.
Beginning in September of 1980, he did several interviews in promotion for his first new album in five years, Double Fantasy. This is (probably) a comprehensive list of those interviews (the date is when the interview was conducted, not published):
September 1980: Playboy magazine, interviewed by David Sheff, spanning several days over the course of multiple weeks.
September 18, 1980: New York Times, interviewed by Robert Palmer
September 24, 1980: WGRQ-FM, Buffalo, New York, interviewed by Lisa Robinson
September 25, 1980: Newsweek magazine, interviewed by Barbara Graustark
October 10, 1980: Los Angeles Times, interviewed by Robert Hilburn
November 1980: Esquire magazine, interviewed by Larry Shames
December 5, 1980: Rolling Stone magazine, interviewed by Jonathan Cott
December 6, 1980: BBC Radio, interviewed by Andy Peebles
December 8, 1980: RKO Radio, interviewed by Dave Sholin and Laurie Kaye
Every one of these interviews but those for Esquire and the New York Times were also recorded on audio or video tape and known to exist. Esquire and/or the New York Times interviews may have been recorded, too, but if so, it's never been revealed publicly. The rest of them have either been released/broadcast officially, or else have leaked over the years, in whole or in part.
At least two of them are confirmed to have been released in incomplete form:
A few brief excerpts of the Rolling Stone interview have been broadcast over the years, in addition to the text of the original print interview. In 2010, interviewer Jonathan Cott reported that he had located the full nine-hour interview when going through his belongings, though this has not seen the light of day.
Bits and pieces of the Playboy recordings have been used regularly over the years in documentaries, including in the 1988 film Imagine: John Lennon and the 1995 series The Beatles Anthology. In all, a couple of hours total have been broadcast from these tapes. In addition, a lengthy transcript of this interview was published as a paperback in 1981 under the title The Playboy Interviews with John Lennon & Yoko Ono, later republished under the title All We Are Saying. However, the transcript is known to be incomplete, because many excerpts were broadcast during the 1988-92 run of the Westwood One radio series The Lost Lennon Tapes, and many excerpts are much longer than what is found in the book. But exactly how much is missing from the transcript is unknown. The audio tapes of the full interview are reported to run over a dozen hours.
The Newsweek interview probably also exists in a much longer form, since Graustark played back excerpts of it on a radio show for WNEW-FM, and it is clear she had more that was not played. In contrast, the interviews for WGRQ-FM, RKO Radio, and BBC Radio have all been broadcast in their complete form.
Anyhow, in none of these interviews was Lennon ever asked about "Wonderful Christmastime," nor did he volunteer any opinion on it. Perhaps it is hidden away in unpublished portions of the Playboy, Rolling Stone, or Newsweek interview. But if so, no such information about "Wonderful Christmastime" has ever been reported.
Many interviewers did ask Lennon questions about McCartney, but it was mostly in the context of the possibility of a Beatles reunion, or else what the present status of Lennon's friendship with McCartney was.
About the closest Lennon got to addressing your question is in an unpublished portion of the L.A. Times interview, which was conducted at the Hit Factory music studio in New York City, and preserved (at least in part) on VHS. This has leaked on unauthorized releases over the years. Here is a transcript of the relevant portion:
HILBURN: Do you keep up with Paul's albums at all?
LENNON: No, I dropped out around the one where there's a rose in his mouth--
HILBURN: Red Rose Speedway? [Note: released in 1973.]
LENNON: But he had the singles. It's on the radio out here. And, uh, I thought "Coming Up" was great. And I liked the freak version he made in his barn better than that live Glasgow one. You see, I'm--
HILBURN: Going to something out[???]. The one with the speeded up voices?
LENNON: Yeah, I think that's [it]. If I’d been with him, I would have said, "Yes, that’s the one," too, and I thought that the record company had the nerve changing it round on him. But you know, I know what they mean. They want to hear the real guy singing. But I like the freaky one. Stevie Wonder does it [and] nobody moans at him.
HILBURN: Had you stopped listening to him, or did you--. Was there a sense of competition?
LENNON: Well, he was putting out so much stuff. I couldn’t keep up with all that tingly-tingly stuff going on albums. You know, they’re all just dribbling on.
From there, the interview turns back to the early 1970s and the beginning of the ex-Beatles' solo careers, before moving on to other topics.
Considering that Lennon had heard McCartney's most recent single (off his 1980 album McCartney II) on the radio, and appeared to also be familiar with the A-side and B-side of the single itself, it's almost a certainty Lennon did hear "Wonderful Christmastime," too, considering the heavy airplay it received. But whether Lennon considered it among McCartney's "freaky" stuff, with its driving synthesizer riff, or among McCartney's "tingly-tingly stuff" is unknown.
Unless Sheff, Cott, or one of the other above interviewers has a tape hidden away with a question posed to Lennon about "Wonderful Christmastime", then Lennon's opinion on the single may be lost to time. I could find no second-hand account in any published source of what Lennon may have thought about the song, either.
As far as I can tell, John Lennon never specifically commented on 'Wonderful Christmastime'. He only had about a year to do so; 'Wonderful Christmastime' was released for Christmas in 1979, and Lennon was shot in December 1980. TL;DR, I think it's reasonably likely that Lennon actually never heard 'Wonderful Christmastime'.
In the earlier post-Beatles years, Lennon was often more disparaging of Paul McCartney, most famously in the song 'How Do You Sleep?', which had a bunch of lines like 'the only thing you done was Yesterday/ and since you're gone you're just Another Day' (referring to McCartney's biggest hit and his most recent hit), and the line 'the sound you make is muzak to my ears'.
However, by 1980, after several years of a retreat from public life, John Lennon's views on McCartney were softened a bit compared to what he was saying in the early 1970s. In a (very lengthy, book length) interview with David Sheff of Playboy in September 1980, Lennon said of McCartney's solo output that:
LENNON: ...I haven't seen any of the Beatles for I don't know how much time. Somebody asked me what I thought of Paul's last album and I made some remark like, I thought he was depressed and sad. But then I realized I hadn't listened to the whole damn thing. I heard one track... the hit 'Coming Up,' which I thought was a good piece of work. Then I heard something else that sounded like he was depressed. But I don't follow their work. I don't follow Wings, you know. I don't give a shit what Wings is doing, or what George's new album is doing, or what Ringo is doing. I'm not interested, no more than I am in what Elton John or Bob Dylan is doing. It's not callousness, it's just that I'm too busy living my own life to be following what other people are doing, whether they're the Beatles or guys I went to college with or people I had intense relationships with before I met the Beatles."
PLAYBOY: "Besides 'Coming Up,' what do you think of Paul's work since he left the Beatles?"
LENNON: "I kind of admire the way Paul started back from scratch, forming a new band and playing in small dance halls, because that's what he wanted to do with the Beatles... he wanted us to go back to the dance halls and experience that again. But I didn't. That was one of the problems, in a way, that he wanted to relive it all or something... I don't know what it was. But I kind of admire the way he got off his pedestal. Now he's back on it again, but I mean, he did what he wanted to do. That's fine, but it's just not what I wanted to do."
It's interesting that Lennon singled out 'Coming Up' as a 'good piece of work'. This was also a critically reviled song for quite a while, a bit like 'Wonderful Christmastime'. It was from the same solo recording sessions as 'Wonderful Christmastime', which was recorded at the same time as the McCartney II album that 'Coming Up' is from. So, potentially, Lennon may well have liked 'Wonderful Christmastime' if he had heard it. (I think 'Wonderful Christmastime' is unlikely to be the song that Lennon thought McCartney sounded depressed - 'Waterfalls', maybe?)
But, honestly, I think it's relatively likely that Lennon simply did not hear 'Wonderful Christmastime'. If he was telling the truth about not really following the music of Elton John or Bob Dylan or George Harrison - and he probably isn't, because he at least wrote a song called 'Serve Yourself' in response to Bob Dylan's 1979 'Gotta Serve Somebody' - it does stand to reason that he would not be listening to every single that McCartney put out (elsewhere in the Sheff interview, he discusses his relief at not having to churn singles out the way that McCartney does).
That he never heard 'Wonderful Christmastime' becomes more likely when you remember that the song, while a decent hit in the UK, was a much less successful song in the US, reaching only #28 (whereas 'Coming Up' was a #1 in the US) - edit: this is the main thing where I disagree with /u/texum's reliably excellent post which I didn't see before I posted this. How many songs that only got to #28 in the charts this year did you actually hear unless you were deeply engaged in current pop music? Lennon during this period was living in New York City, at the Dakota Building, and so his media diet would likely not have included 'Wonderful Christmastime' with quite the frequency that he might have experienced if he had still lived in Liverpool or London. So it is very possible that Lennon simply had a wonderful Christmas time in 1979 with Sean and Yoko in New York, blissfully unaware of the McCartney song. And because the song, because a Christmas song, would have been removed from playlists after the holiday was over, Lennon genuinely might not have heard it if he really was not focused on current pop music (as many people who are near 40 years old do not).
Of course, there may be some other outtake of an interview where Lennon is doing publicity for Double Fantasy where he does discuss 'Wonderful Christmastime' that I'm not aware of. Or perhaps Yoko Ono has in her archives a playful parody of the song, 'Serve Yourself'-style. But I think it's more likely that he had simply never heard of it, or was never asked.