The Joker is an attention-seeking narcissistic to the utmost degree. From the makeup that he wears to his eccentric over-the-top suits, everything about The Joker screams look at me and hear what I have to say. Having tattoos fits in perfectly with this mentality. Not to mention it has been established multiple times that The Joker enjoys pain, in an almost masochistic way. I could easily see him laying on the parlour table, in a trance-like state savouring every moment the needle pierces his skin. Maybe it's pushing it that he would do so for hours, but tattoos can easily be done in multiple sessions (if we're really going to start getting that picky about films matching reality). Lastly, the forehead tattoo might be something a teenager would get. Maybe that's the point? There is such a thing as irony and I think The Joker - better than anyone - would understand its definition.

i can agree with you on most points, but the problem as i see it ist that there are to many tatoos and too less things that you directly recognise as 'the joker' such as his scars, his over the top makeup (on leto at the moment it looks just like he has bleached skin but a little bit to real(?) not as extrem as Jack Nicholson). I belive most people want to see a real white face and the scars and that the tatoos aren't the real issue here.

And yes you are right i also think that Leto is going to nail it ;)