You'll probably want to expand your query to include the opinions of jazz guitarists, since jazz musicians were vocal participants in the many elitist responses to rock and roll. In many ways, they were actually the most vocal opponents to rock - generally speaking, leadership in the jazz world was growing more conservative than leadership in the classical world as rock and roll ascended, and the harshest words of criticism against it usually came from the Bebop ranks.

Complicating our search for the opinions of guitarists is the fact that prior to rock's ascent, guitar was nowhere near the ubiquitous instrument it is today, and had somewhat limited professional applications and very few "superstars" whose opinions would be considered worth publishing in major trade publications. "Illegitimate" genres like country, rock and roll, and jump blues are part of what popularized the guitar and made it so ubiquitous; prior to the 1960s it was simply not the most common instrument.

Most primary sources I could find on this (such as the 1956 Downbeat article "Musicians Argue the Value of Rock and Roll") featured brass players, wind players, bassists, and pianists. Guitar was still something of a niche or supplemental instrument and not yet the staple it became. After The Beatles played Ed Sullivan, guitar manufacturers had to up their manufacturing output practically overnight to keep up with demand. (There is a rather incredible comparison between guitar sales pre-Beatles and post-Beatles, but I can't seem to track it down at the moment.) So finding the opinion of a guitarist will be much harder than finding the opinion of wind or brass players, who could outnumber union guitarists as much as 8:1. In the world of classical and theater orchestras for the period in question, a guitarist was as much or less in-demand than a harpist.

While you asked specifically about classical guitarists, I'll give some info about how musicians in the industry leadership reacted as supplement, as that's close to the nature of the question. The American labor union American Federation of Musicians (AFM) generally sought to exclude rock and blues musicians from their ranks, which weakened their union's standing tremendously over time as rock and roll ended up fundamentally changing the industry landscape.

(As a side note, this debate was raging just after or just as musician's unions were de-segregating, and opposition to rock was often most strongly voiced by black union leaders and members such as Chicago's William Everett Samuels. One wonders between the black union leadership response and the jazz response (and the recent southern origins of rock, and the efforts of bebop musicians to distance themselves from their Dixieland predecessors) if there wasn't a bit of North-versus-South or "talented tenth" elitism at play here. This is speculation, but I think it would make for fascinating scholarship.)

AFM required all members to be able to competently sight-read a piece of repertoire in an audition. While rock musicians are not nearly as universally untrained/illiterate as laypeople assume, the repertoire was usually classical or jazz anyway, and the auditions also required interpretation - so even a musically literate rock musician unfamiliar with certain genre conventions might bomb the audition. This excluded a huge portion of working musicians from the union rolls, until eventually the vast majority of paying non-orchestral and non-theatrical live performances ("club dates") were being played by non-union ensembles.

This meant that when the National Board of Labor Relations in 1984 took away the ability of the AFM to negotiate club rates on behalf of all musicians (including those not in the union), gig rates plummeted and haven't come back since, as the "second-class" non-union musicians had lower pay standards and were in higher demand anyway. You'd probably be surprised to learn the extreme effects of that ruling: an unknown 4-person band playing the Whiskey A Go-Go venue in the mid-1960s would have made better money (in absolute dollars) than a band today, to the tune of several hundred dollars.

However, while the union was for the most part a monolith, its ranks weren't. A sizable minority of musicians saw the writing on the wall and (accurately) predicted that continuing to bar some of the best-paid and most frequently employed musicians from their organizations would weaken their collective bargaining stance. George Cooper Jr, the president of Nashville Local 257 (the national union AFM is organized into several smaller organizations per region called "locals") was the first to remove the audition from union requirements, which led to tremendous growth for the Nashville chapter. Unlike more exclusive chapters like Chicago or Los Angeles, 257's membership rolls included country, blues, and rock musicians and was a vibrant union during a time of nationally diminishing power. Cooper was an outspoken opponent of what he viewed as elitism in the AFM.

Still more union musicians were more than willing to cash checks playing rock sessions and writing arrangements for them, even if often they didn't hold the music in very high regard.

In general, you'll find that the responses of musicians to new trends in their field will be very diverse, and you'll have a healthy mix of early-adopter enthusiasts and vocal reactionaries, as well as plenty of people who will gladly cash a check and not commit a word about it in writing either way, because a gig is a gig. I would imagine that many adaptable guitarists found themselves in the latter category, as an explosion in "low-brow" guitar-centric music meant a consequent explosion in gigs for trained union guitarists who could ape those styles for all sorts of commercial gigs - film scores, jingles, theater pits, etc.

Main source: Tell Tchaikovsky the News: Rock ’n’ Roll, the Labor Question, and the Musicians’ Union, 1942–1968 by Michael James Roberts

Edit: so many edits, because I am on mobile and because I no learned to write good when I was in the music schools.

Jazz players and arrangers were all over Jimi Hendix's Music. Everyone from Gil Evans to World Saxophone Quartet arranged and performed his stuff. How did classical musicians think about rockers? They wished they could be paid as much and had as much attention for their performances. It's show business and even though a classical musician is much more proficient on their instrument than all but a few pop musicians if no one wants to see the show disdain for a succesful performer is irrelevant.