It seems strange to me that this would be a common practice, I would suspect it to be an extremely rare occurrence done by the most messed up soldiers, but perhaps I'm wrong.
Almost certainly, that person was referring to this rather famous photo from Life magazine taken on Guadalcanal in 1942:
http://i.imgur.com/Zt6AWaN.jpg
Time magazine interviewed the photographer, Ralph Morse, in 2014, and he gives an interesting account of the photo. Morse said that:
The sergeant leading the patrol looks at it and says, ‘Guys, that skull has been put there for a reason, and the Japanese have probably got mortar shells aimed right at this spot.’ A disgusting scene like that will always draw people in, and the idea, of course, was that any American troops who came along would obviously want to stop and take a look.
“‘Everybody stay away from there,’ the sergeant says, then he turns to me. ‘You,’ he says, ‘go take your picture if you have to, then get out, quick.’ So I went over, got my pictures and ran like hell back to where the patrol had stopped.”
Honestly, I'm not sure if I buy this version. Like later island campaigns, the Guadalcanal landings did not meet much in the way of initial resistance. The Japanese retreated to higher ground. It is hard for me to believe that Japanese soldiers were able to move unmolested on the beaches at any time during the battle, much less coordinate with artillery to zero in positions on disabled American tanks.
Here is a map of US positions on August 12. I just don't see any way Japanese infantry get close to Lunga Point.
I tend to believe that the sergeant, likely embarrassed in front of the Life photographer, invented that story, knowing full well it done by US soldiers.
As far as your question though, I don't think you can say that the practice of decorating tanks with heads "wasn't uncommon." I don't know of any other photos or stories of this occurring. If it did occur somewhere else, I would still describe the practice as uncommon, if not almost unheard of.
This is not to say that US troops didn't do some extremely morbid things. Just that this one is seems exaggerated from a photo of uncertain provenance.
EDIT: Just for the sake of completeness, I think it is worth mentioning the US soldiers did have a documented practice of using Japanese skulls as a warning sign for dangerous areas, as shown in this photo taken on the island of Peleliu. And there are several examples of skulls being used for decoration in the pacific, just not trophies on tanks.
