It's just a joke he makes whenever any cartoon character has a stonetoss face. He did the same thing with some cartoon network show.
Also, I really enjoyed smiling friends and hope it gets a second season. Zach mentioned in a podcast about the production of the show that the two exerted creative control over basically every moment in the show (which is a lot more oversight than the vast majority of show creators employ) and it really shows with the attention to detail in terms of things like the dialogue and background gags. For example, mip's bomb looking like a legit homemade bomb that someone like the Unabomber would have made is an underrated part of that bit.