That lens is quite good actually for close-ups - I have used it in the past for close stuff and been fairly happy with it.
Where did you focus on the mantis? It looks slightly sharper on the little thorny things on its leg than it does on the face/eyes.
I focused on the face but it kept moving but that's the problem with shooting in the " wild " . I have to say I do like my 24-105 it performs well whatever I use it for I would recommend one to anyone.