Judaism says no tattoos; it also prohibits all piercings except earrings.
That's hardly a universal interpretation among Jews. More common than among Catholics, but there's extensive debate in both circles over whether Leviticus 19:28 relates to blasphemous tattoos, or all of them.
So basically I read that while the Roman Catholic Church does not prohibit all tattoos like some fundies (and I'll give it credit for that) it does have guidelines you can't have any "indecent" ones or ones with religious imagery. No crosses, pictures of Jesus, etc.
So if true, why? Seems odd.
Well, its not true, so lets end it there... unless you put Jesus' face on your ass, or something.
Though, I will take this moment to praise you for asking, for once, as opposed to simply assuming.
To be fair, a minority of Catholics do hold that view, and there's no Church documents I could fine. So I'm not sure you can really outright say "it's not true"...
If the Church hasn't bothered to release anything on it, I assume it's a rather trivial matter to them either way.
(I don't want to sound condescending, obviously you kick my Catholicism-understanding ass to the moon.
)