Tattoos are pretty taboo in Japan, so for him to think it's cool on a girl means he's a mad lad. And it's ironic considering his very Japanese outlook on delinquents. Interesting...
@humbleoverlord I don't know about other cases...but, here it's justified. Yankii girls are known for and have a reputation for getting pregnant and married at very young ages.
I don't really agree with the sentiment that the MC is dense or can't pick up any hints. It just genuinely looks like he's not interested in her like that (as of this chapter).