I think you need to take your cue from the person with the tattoo, wether they are happy with it or not.
If they are proud and very happy with it and you think it's the ugliest or weirdest thing, well "How unique"/"how interesting" is a start (and can be an end).
One of the easiest tattoo to butcher (it seems) is a portrait, so a "the subjet must be touched (to have her face inked for eternity on you)" or "It's sweet/touching to always have her as a memento" (for a tat of a deceased person for example).
If they are lamenting it can go from a simple "oh, that sucks" to trying to reassure them "I'm sure people will think the they/their mistake was intentional and so clever!"/"it doesn't look that bad, really

". It all depend on how close you are and how much you want to talk about it...