Ambrose has been doing his thing as an unhinged character before Dark Knight. This promo was from HWA Unbreakable, which took place three days prior to DK hitting theaters.
Most of his best stuff was post-DK so take it for what it's worth, but I'm not sold that it's a given that he watched the movie one day and thought to himself, "Hey, I should do that." It's easy to see the comparisons, but he's got some Piper and Pillman to him as well. But ultimately Moxley was being Moxley/Ambrose being Ambrose.