You know what X? is a scheme for opening a conversation, or a new segment of a conversation:
You know what I hate / think? You know what I’m thinking / gonna do? You know what scares / annoys me? You know what pisses me off?
Framed as a question, but not seeking information; after all, how could the addressee know what’s in the speaker’s mind?. The speaker is going to answer the question, in any case, and the most that’s expected of the addressee is an encouragement for the speaker to go on: No, what? or something of the sort.
And then there’s the bonding of men through mutual insult and contention, giving in this case “new best friends”, as the strip’s title says.
As for Lippy, the Zippy site tells us that he’s
Zippy’s twin, yet diametrically opposite, brother. Lippy dresses in black and thrives on misery– his own as well as others. He only enters Zippy’s life for one purpose: to try and make him unhappy. Good luck, Lippy.
Here he’s out on his own, where he comes across the sociopathic Mr. the Toad.