I don't necessarily see the depiction as The Joker as racist (except, perhaps, in its clownish resemblance to minstrel-show makeup), but I do see it as grotesque and as an over-the-top accusation that Obama is some sort of evil villain.

Critical comment is fine when it's reasonable. This is not reasonable, and it's not even really "critical comment" -- it's just nasty. Moore's assertion that his banner was only "intended to show his displeasure with Obama's policies... not [any opposition] to the president himself" is ludicrous.