This is, in my opinion, tricky ground here.
Personally, I think that if you go back 50 years and look at the tattoos that are common then you'll see a lot of mismatched tacky tattoos. They have meaning to their owner, but they are - in my opinion - tacky to look at.
In contrast, today there are still a lot of tacky tattoos. There are a lot of cliches for tattoos, there are even some that are rather trashy. But a lot of that depends on the person, the tattoo, the location. And what to me seems to be a tacky or a trashy tattoo might have great meaning to the person who has it.
At the same time, there are tattoos that are obviously symbolic - the navy tattoos, the religious ones, the ones someone's name or birthday or what have you on them.
And then there are tattoos that are purely about the art. I've seen tattoos that are really elegant masterpieces etched onto someone's body. It might not be how I would express myself, but I've seen some women who are very tattooed and their tattoos add to their beauty.
I don't think Sofia's tattoos are ugly or trashy or whatever word you (or anyone else) want to use, but that's my opinion. I do think that it's her body and it's her decision how she modifies it. I think that as far as tattoos go it's not that bad - it's smaller, it's on her back, it's easily covered (if she wishes to cover it) by her hair or clothing. It's not like she has a giant sleeve down her arm or a Mike Tyson tattoo on her face.
My comparison to Stephanie of Monaco was more that there are other princesses out there who aren't a part of the main line who have tattoos that are just as visible as Sofia's in formal wear, but the institution of the monarchy didn't come crumbling down because of it.