1) The boyfriend seems to think that talent shows are stupid. (This is mostly clear from his tone of voice.) Mimes are generally regarded as annoying, so most people wouldn't want to be in a talent show that includes mimes. Magicians can be popular if they're really GOOD, but "talent shows" in general are known for often having a lot of very silly or bad performances. I think the boyfriend is simply mentioning a couple of juvenile-sounding but typical "talent show" acts to suggest to CJ that she shouldn't participate in this kind of event. He's saying that she'll be performing in the middle of a show with a lot of silly or "lame" performances.
2) This seems to be slang for "let's go" or "let's leave." (I've never personally heard anyone use "let's hit it" this way, but given the context, that's clearly what he means.)
3) No, it's not idiomatic. Dogs like to roll around in stuff on the ground, especially droppings left by other animals. The show writers are trying to say that the dog wants to roll around in something disgusting (as dogs usually do), but in order to make the line funny instead of disgusting, they're leaving it up to the audience to imagine what kind of "stuff" he probably wants to roll in.