I told a guy I thought his tattoo was shit when I was out drinking one night. He actually got really sooky. Then half hour later he walked up to me at the bar and threatened to punch me. He wasn't very big so I told him to try it and I'd send him home in a wheelchair. He stormed off and then his girlfriend approached me and berated me on the rudeness of insulting someone's tattoo. Then they both left the club.
I ruined their whole night. What a soft cock though, seriously.
He had this, which I think looks ridiculous (It's a symbol of some band, HIM, who I think suck as much as their symbol):
So anyway, point to the story is, most people compliment tattoos because they are something usually personal to the person who has them and insulting their shitty tattoo makes them feel bad.