It really doesn’t matter two hoots whether any of us like it or not, does it?
Everyone grieves in their own way, everyone keeps mementoes or not, and this particular person presumably finds the tattoo comforting.
I kept a lock of DH’s hair to stroke. As a Christian I know his soul is with God, and his mortal remains are buried in our churchyard, but I just wanted something physical to hang onto until we are reunited.