Heteronormativity is when everyone assumes that everyone else is straight by default. It also normalises behaviors that are considered “straight”.

Toxic heteronormativity is when someone’s heteronormative views makes them act in a toxic way (ex: “if my son is gay, I’ll kick him out”, or stuff like “A guy must like having sex else he isn’t normal”)

So no, it’s not mocking straight people for being straight, but mocking the silly behaviours that come from people taking the stereotypical role off husband and wife way too far.

Hope that explains it better :)