It could also be attributed to the fact that Axel may have had to lay low during CoM. As we know, he wasn't exactly on the same track as Marluxia and Larxene... also, different characters may bring out different sides of his personality. We've seen him with Roxas, Kairi, Sora, Saix, etc. all of which he reacted differently to. Perhaps we just saw the side of him that Marly and Larxene bring out.
But personally, I thoroughly enjoyed his performance. Besides, to expect him to act in CoM the same way he did in KH2 doesn't make sense... apparently, a lot happened in that year we're waiting to see. And wasn't it said somewhere- I think in a novel translation- that Axel was bored out of his mind, to put it simply, until Roxas showed up? Maybe the monotony of his life was projected in his tone.