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.

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/

they are statistically cooler

Which is a strong confounding factor ...

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

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.