What is the eligibility for IAS or KAS if I have a tattoo?

I don't think there is any problem as your physic doesn't matter in it. But, in the interview, panel may ask you about it.

Hi, Physical test is not conducted by UPSC. An affiliated hospital will do that and they are good people. Unless you have some real health issues, they won't reject you. They will have in their hand a form that contains various parameters. That form doesn't contain whether a candidate has a tattoo