The pants in a [something-y] pants is just a way of turning an adjectival description into a noun.
The more common phrase, one that has an actual dictionary definition, is smarty-pants:
: SMART ALECK, KNOW-IT-ALL
"If you give people the impression you're a smarty-pants, that's no good for sure."
— Al Gore
So, a smarty-pants is somebody who is smart (or trying to be), and the expression is used in a kind of derogatory or sarcastic sense. ("Oh, look who's so smart . . .")
A growly pants would, similarly, be turning the adjective growly into a noun phrase so that it can describe the person. (In this case, it's somebody who growls—not somebody who is grumpy or, necessarily, complains a lot.)
The additional use of Mr. does the same thing. It could just as easily be Mr. Growly, Ms. growly pants or, to make up a different noun phrase, Mr. eats-a-lot (for somebody who is known to eat all the time).
Also according to Merriam-Webster, although it doesn't give an actual source, it says:
The first known use of smarty-pants was in 1932.
No doubt other uses of [something-y] pants come from simply changing the adjective from the well-known smarty (in context) to something else.
Note that there is also fancy-pants, which Merriam-Webster says originated seven years later, in 1939.