When Fantasia got rereleased around the Hippie days, someone asked one of the animators if he used drugs, to which he replied "Yes, I was on drugs- Ex-Lax and Pepto Bismal".
Regarding Ralph Bakshi and drugs:
From
http://www.blackbookmag.com/article/ralph-bakshi-on-the-fritz/2454
I also once read that Ralph was such a workaholic when he did the 1960's Spider-Man cartoon series that both his girlfriend (who I think was actually his first wife) and his drug dealer left him. So, I can conclude that Ralph Bakshi did experiment with drugs, but in the end he didn't like 'em, and it most likely didn't affect his creatively in any positive way (at least, I doubt Ralph would believe it did). And I really don't think John K. ever did drugs, because there really isn't any proof. The Mighty Mouse thing isn't strong IMHO. If evidence popped up one day to support the notion that John K., at one time or another, used drugs, I'd be open to it, but until that day comes, I'm convinced that John K. has never done drugs.
Kevin Smith wrote the episodes of the Clerks cartoon, and he has a history with marijuana, but he didn't really become a regular pot user until 2008 (when he worked with Seth Rogen on Zack and Miri Make a Porno), nearly a decade after the Clerks cartoon was canceled.