Tattoos are so common these days that I'm not sure you can say much about a person just because they have tattoos.

Any given person, for sure. In aggregate though I bet you'd find correlations.

they are statistically cooler

My tattoo artist friend who spends a large part of her days tattooing Chinese characters on people who don't care what they mean would beg to differ.

She is also an incredible artist who has won several awards for her custom work. But when someone walks off the street and picks a random design from the book, it's her job to do it.

Which is a strong confounding factor ...

"Sharing traits common in motor cycle gangs increases risk of cancer - new study"

Not sure you could in Sweden. Stats[1] show 21% tattoo prevalance in Sweden compared to 12% in the EU in general.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10163470/

No, it would control for a lot of those factors. Things like impulsivity, sexuality, sensation-seeking etc. All very heritable or family-level. If you ever find any correlations reported for tattoos between family members, it's gonna look like everything else: steeply increasing with relatedness.