I think it all really depends. Both "tattoos" and "piercings" are very wide groups.
An ear ring is completely normal for most girls (heck, most I know around here get their ear piercings before they're 5 years old), not really comparable to someone with 25 piercings through their eyebrow or something.
I'm not a big fan of piercings in and of themselves. Some can be usedto create nice aesthetic effects. I quite like the sort of piercing on the top of the ear, with a thin chain leading to a second piercing on the bottom of the ear (Bajorran-style, yes
). Nipple piercings can be fun to play with for once but I wouldn't want them on my girlfriend. Belly button can, again, be quite nice and can be used in combination with a top showing the belly to make some nice visuals.
Tattoos vary even more for me. My sister-in-law's husband has a large tattoo on his back, of a dragon with the names of his two children worked into the lines of the wings; the whole designed by his wife. It looks nice, and has emotional value to him, being a reminder of his family and such.
A friend of mine has a small tattoo with the names of her two living children, and a smaller one with the name of her third child who died during labor - they represent quite an emotional investment for her as well.
Besides that kind of emotional appeal, I can understand people who have a specific way of life" - who have a hobby that's really a complete part of their life - taking on some image of that hobby or lifestyle to reinforce the lifelong bond. I personally don't really go for it, but if you're just *that* much into Zelda/Mario/Madden '14, well, I'm not judging. With these, I think the aesthetic value starts to really matter as well, though. I think it's Hylian who just got a Link tattoo? Anyway, that one's clearly a way to express his personal strong bond with his gamer roots, and with Zelda/Link etc. Fine by me, but it's also just a good looking tattoo.
One step further are people who get tattoos just for the looks. The right tattoo can be sexy and/or beautiful in the right spot - flowers circling a leg, lines from the back to the breast, whatever. I've seen true pieces of art, and I certainly don't think using the human body as a canvas makes it any less artistic than painting on...err, actual canvas.
Of course there's also the "badge of honor" style tattoos - whether it's from an army deployment, a marine unit, a gang or the prison ward you were in, those are signals of who you are, who you identify with - groups or people who've become an integral part of your own individuality.
And lastly, there's dumb fucks putting ugly things on their body for no good reason - the type of "Entry here --->" above the ass and such. Those are just silly.