It's not just Iron Man. Deadpool has a huge "cult" following (so to speak) and there have been months where several titles had Deadpool variants all at once (if I remember correctly, esp. when X-Men Origins: Wolverine hit the theaters).
So he's been on the covers of . . .
Avengers - http://tinyurl.com/kfbd2y2
Guardians of the Galaxy - http://tinyurl.com/jwumo2v
Wolverine: Savage - http://tinyurl.com/k9gpuj9
Amazing Spider-Man - http://tinyurl.com/l9dbe4o
etc. etc. etc.
Not only is he popular, but he's also one of the only Marvel comic characters to break the 4th wall (http://tinyurl.com/m4dveko), so it fits his modus operandi (sp?) to photo bomb the covers of other characters and steal their spotlight, so to speak. He's the one who actually knows fans are out there watching them!
Hope that's helpful. It's not just an Iron Man thing.