Beauty may be in the eye of the beholder, but that has never prevented most people from seeking out some form of larger, group consensus. Think about how often you ask someone else what they think of clothing you wear or how your hair looks. Have you ever turned to a total stranger in a store and asked their opinion on a color match on some items? Although they are never going to see your house and you have no idea who they are, you still asked, didn't you?
When it comes to body art and tattoos, it's an interesting mix of what makes or breaks an art choice. At the most basic level, the technical execution of the body art. Do a pair of piercings display bilateral symmetry (meaning do they line up the same)? Is something crooked, or too high or too low? One of the biggest complaints people have when it comes to piercings is poor placement. Pairs of earrings or nipple rings are expected to match up as closely as do the parts on each side of the body. This is a huge issue for nose piercings, so by all means, have the piercer put a dot on your nose with a permanent marker and make sure you like where it sits. If not, just wash it off with a cotton swab dipped in alcohol and remark until everyone agrees.
For tattooing, line quality is a huge issue. Is a tattoo line solid or spotty? Is an outline wobbly, or shading really splotchy? These are some of the basic technicalities to tattoos. Some artists work with stencils, some swear by their ability to freehand a design. It's best that a client do as much research into an artist's style and make sure they are comfortable before going under the needle. And when it comes to spelling mistakes, it's a terrible client-said, artist-said tug-of-war. Clients should check the design and any skin transfers for spelling mistakes before work starts and artists should have the balls to tell a client when they've brought in something misspelled and not just "tattoo what they asked for." And then if it's still spelled wrong, it's a mutual lack of intelligence.
Then there's the social opinion of what you just did to yourself. Human cultures have varying degrees of conformity and aesthetic senses. No matter what you do, you'll find people who like how it looks and those who think you've just disfigured yourself for life. Those practices which appeal to a majority are sanctioned (plastic surgery) and those that aren't sometimes find themselves relegated to fringe culture (extreme body mods). It never hurts to take a moment to consider what you are about to do and where it falls on the larger scale.

