Yes, the younger Rick is awkward on camera, he remains stonefaced amidst chaotic scenes unfolding around him, and some of his antics don't feel believable.
But guess what, that's the charm and absurdity of his character and storyline. It actually works here, because 15 yr olds are awkward, and non-reactive to shit when they simply don't grasp the gravity of their actions.
I don't think the film suffered at all from the lead's inexperience. It actually lends itself to the film. Now would I want to see buddy in a different film with the same chops? Probably not.