In the first example you give, someone who says “a tattoo on girls” MAY be trying to make it clear that they are NOT talking about girls who have MULTIPLE tattoos. But the best way out of this is to make BOTH terms singular — “I like a girl with a tattoo.” And of course, “I like a girl with tattoos” is fine.

As for “girls in a skirt”. It is incorrect, but I would not make such a big deal of it for casual conversation. (Note that in this case, there should be no concern about using the singular “skirt” to avoid confusion. since it is extremely doubtful we’re contemplating a girl wearing several skirts at the same time!)