Akainu melted part of his head and seemingly his leg. This is significantly worst than pretty much any beating any other character can achieve because fighting Akainu leaves you permanently damaged if not mortally wounded.
Just name one character who with such "average" attacks makes so much damage to people as powerful as Whitebeard (melted half of his head, burned his organs), Kuzan (destroyed his leg, left most of his body scarred) and even Ace (pierced a hole effortlessly through his body).
Akainu is dangerous as hell and thinking that what he just did to Kuma is a failure is absurd.