I wouldn't really interpret this as mocking but rather two content creators having light hearted fun about the situation.

If they ARE mocking anyone it's the autists that are constantly getting worked up over modern cartoons and blowing shit out of proportion than people that are actually constructively criticizing said cartoons

I hate when this happens
like,

Fan of show: brings up genuine criticisms toward a show to the showrunner on twitter

Showrunner: makes a silly drawing mocking their criticism HAHAHA I WIN AGAIN proceeds to do next to nothing with their show art wise

(Note: I'm not saying gravity falls has bad artwork or animation despite the now overused artstyle, quite the contrary in fact, I'm just making a general statement)