Did Kurt Vonnegut do a lot of acid or was he just like that?

It's hard to tell. In the book "Breakfast of Champions", where he pretty much just spoke his opinion about how stupid the world is, he spoke negatively of drugs, ofcourse in that case he was more talking about stuff that wasn't meant to be used a drug.

However i personally find it hard to believe that he would be able to invent a book such as Slaughterhouse Five, which is about aliens who see all of time at the same time, with out aid of drugs.

I personally suspect he may have used LSD and Cannabis at one point or another, or perhaps the whole time. Good on him for using them. He wrote brilliant books.