I'm sure some doctors and other people might tell you that because they are personally against tattoos
But it's certainly not true that you cannot get one just because you have UC. I've had UC since I was 14 and I have gotten 7 tattoos since turning 18 :) They're addictive!
However I did get one on prednisone once and it didn't heal very well, just needed a bit of a touchup after. Just take care of it like they tell you to and you should be fine. Enjoy!!