I'm convinced the "mumbling teenager" trope goes back more than a few decades. Usually used to indicate the character is lacking in self-confidence generally or just not proud of themselves with regard to something a bit more specific.
e.g.: - Gran enquires as to how well they are getting on at school/have they got a girlfriend/boyfriend yet?
Teenager responds with "wulllwudvebtmmmmbbblbllbbermmmumbblewurr", which - in all probability - denotes failure to live up to gran's expectations.
The cognitive effort of simultaneously producing a stream of words without hesitation (where hesitancy leads the average listener to deduce that a lie is being formulated) whilst also thinking up excuses for aforementioned failure is enough to overload the typical, preoccupied, teen brain, resulting in muffled indistinct speech.
:O)