I feel like this has covered why obviously bad tattoos are popular, but maybe not so much why well-meaning people end up with just-okay tattoos. To my mind, crafting an outstanding tattoo involves many variables that just aren't applicable to many other types of visual art. It's even more confusing when you add in the variable of personal taste. I work in a bookstore and need to be well-versed in children's picture books so that I can make recommendations, so I see a pretty wide variety of illustration styles that I love, almost none if which would work as a tattoo - you can look at many picture-book tattoos as unfortunate proof of this. There's wonky line weight galore, and colors sometimes migrate, but, if the composition is solid, the looseness adds to the charm of the illustrations. I find myself disliking anything that's too bright, clean and/or cartoonish. Likewise, classic flash, when executed cleanly, looks fantastic on the human body but frankly wouldn't excite me too much outside of that context. Not to mention that you can't, for instance, pick up your local paper (if you're in a metropolitan area with a reputable paper, which is increasingly rare) and read at-minimum competent tattoo art criticism. I think it's at least partly a matter of retraining your eyes and learning how to look, which is something that might elude even a fairly careful person.
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Having said that, if you get more than one iffy tattoo, it's your fault.