I voted "it's complicated" because I don't know enough about Dinklage and his situation to be sure, but I'm having trouble figuring out from whose point of view it makes sense to throw the epithet "pimp" at him. If "imp" is a term he himself uses - whether playfully, ironically, or whatever - then I think it's fair game for a poster. But "pimp"? If this is supposed to be ventriloquizing someone's critique of him for capitalizing on his stature for celebrity etc., then I would say: yes: offensive, annoying and ableist. But I don't know the context well enough. On the other hand, I love the use of the Obama-poster style. If they had just omitted the caption, the poster would have been more interesting IMO - it would then be up to the viewer to decide/ponder whether the color scheme was meant as a straight-up tribute, ironically etc. The caption is puzzling in a way I suspect I'll like less and less the more I think about it.
I look at it the same way I've looked at Dr. House and his cane -- the cane is part if his character and making the occasional pun or remark about being a "crip" is in line with House's character. Tyrion is an Imp and is called one in the books and uses the term himself. He also takes pleasure in being badass and a lady's man. So to create Pimp is just sort of endearing to me. Now, if It were Bran and it said Gimp, that'd be totally different. Or a mural of Hodor just saying Hodor Hodor. But the mural doesn't seem to fixate on him any more or less than other character driven icons and memes that go around -- if it were Arya and it said "Girl. Nobody." I would feel conflicted but in the end I think I'd love it because the subtext is, Girl, Somebody. The Tyrion subtext revels in his canon badassness. But of course I'm abled, so I can only come to this as the daughter of someone like House -- I try, but I am ultimately an outsider.