Clark: Isn't there a limit
to how tight a shirt can be to meet dress code?
He implies it's too tight
even according to dress code?

Lois: Really? Even you? Are you mocking me too?

Clark: Well...
Lois: I never made it to Mexico.
Clark: Right, can this wait until after your post-lunch, Before lunch?

predinner, after-coffee (coffee break?) snack break?
"Post" means "after." "Predinner" means before dinner.

He's just making fun of all her breaks, referring to some of them by way of describing another one.

"after-coffee" would be after the coffee break. All of these qualifiers refer to the "snack break" at the end of the sentence.

Your earlier suggestions are both correct.
Thanks, Avangi!