So I've just got my debrief back from pre-Selection...help me?!?

I don't think they mean that you are y or loud mouthed or they would not have said that you will be a great et to your regiments. He is saying sometimes you joke around when the situation is a serious one and that may not be appropriate. But don't change the fact that you do have a sense of humour because he has said that that will serve you well on the front line and be et in keeping up the moral of your regiment, I think if you are joking when your being shot at that is probably a situation you should take a little more seriously, maybe you should have a chat with who ever wrote it and get them to clarify when they think your humour has been inappropriate.