It's actually amazing but statistically he is. He goes at 58% which ranks 35th at the Giants with only Taranto, Greene, Coniglio and Hopper really below him who are all pretty well known for bad kicking. Dangerfield for comparison goes at 57% but he is a vastly more contested player. For the type of player Kelly is, which is a balanced midfielder, he's a particularly bad disposer of the ball. Basically he disposes at a rate similar to Oliver/Dangerfield heavily contested guys as opposed to at the 63% type range of the more balanced. In comparison a guy like Pendles is at 68% playing a similar role.
It totally fails the eye test as he has a genuinely brilliant kicking action and technique but the stats don't support the eye test at all in this situation. He looks like an elite kick and executes at a very poor level.
57% is not a bad disposer of the ball for a midfielder, it's bang in the middle as average (94th of 174 among mids) and he's almost as average a midfielder in profile of equal inside/outside (104 of 174 for contested possession rate). However the sheer volume of contested possessions would explain this, he's 46th for average contested possessions, so he still kicks a fair bit under pressure and explains the pure efficiency being a bit low.
I think this also has a lot to do with the kind of kicks he attempts versus the other guys. Danger often kicks long to packs (which is an effective disposal as long as your team has over 50% of the players at the contest), whereas Kelly tries the lower percentage kicks, ones that are more spectacular when pulled off.
You'll also find that Dusty has a poor kicking efficiency but you'd have to be pretty brave to say he's not a good kick. You can tell more about who is a good kick by looking at total effective kicks, and Kelly was 11th for average effective kicks, six of the guys ahead of him are wingers.