He might not have been a lousy shot. He might have just been unlucky. Seems like the kid had a poverty gun with a 1x magnifier and a red dot for a sight which was approximately the same apparent size as Trump's head, meaning there's a relatively high margin of error. Also, Trump turned a little towards the shooter just before he was shot at, which quite possibly saved his life.

Assuming the kid fired real bullets, of course. It still could be fake blood and an event full of crisis actors.