Sadly all that proves is that they don't have them, not why they don't. As it stands I can't say if they didn't get tattoos because they don't want them, haven't found a design they liked, that Xavier Institute has a no tattoo policy (which I find highly unlikely if your mention of Berserk is any indication) or there's just no tattoo shop in their town. Nothing absurd in recognizing that when it's left unspoken, we both are free to insert a reason that makes sense to us.
My dude, they live in fucking new york, not in some amish commune.
Your whole argument does not make A LICK of sense, i cannot prove a negative, if the source material doesnt show them with tatoos, if none of their characterizations shows them with tattoos, if nothing in their years worth of comics show any interest whatsoever about that, then they canonically, do not want tattoos, you cant sit there and tell me to "prove it".
Like whatever the hell you want dude, thats not the point, but you cant go around arguing that they should have them because for you it feels that way, they arent like that in the comics, just take that as it is, thats how their characters are, anything else is on you.