As someone pretty involved/familiar with tattoo culture, getting a flash piece off the wall is pretty common and accepted.
I mean obviously you don't walk into Tribal Overlord Tattoo on the strip in Orlando Florida (doesn't exist) and pick a biomechanical alien off the wall, but you would walk into say Smith St. Tattoo in Brooklyn (does exist) and pick something off the wall.
As someone also involved/familiar with the tattoo culture, I'd say that "accepted" is taking it a bit far, and that "tolerated" is closer to the mark. Most tattoo artists I know produce flash because they know it pays the bills, not because they really like people using it. I don't know any tattoo artist that won't jump at the opportunity to create something new and unique.
Drinking at a party, to the point of über silliness is common an accepted. It's still not prudent and you still have to deal with the consequences the next morning (and maybe for the rest of your life in certain circumstances.)
So, it's an "are you prepared to deal with the consequences?" kind of thing. Maybe the teenage girl had already decided she'd pick something small and unobtrusive and had chosen a discreet location for the work. Maybe, and here's where imprudence comes in, she walked by a tattoo shop with her friend and decided "a tattoo would be, like, totally hardcore!" and gave it no more thought.
I mean obviously you don't walk into Tribal Overlord Tattoo on the strip in Orlando Florida (doesn't exist) and pick a biomechanical alien off the wall, but you would walk into say Smith St. Tattoo in Brooklyn (does exist) and pick something off the wall.