Indeed, it acts as a social commentary on holiday overconsumption while simultaneously showing off the comic pedigree of the developers.
Many popular animated series in America (and many other countries) have had episodes where one of the main character gains weight
Homer gains lots of weight in an episode of The Simpsons
Lois gains a lot of weight in an episode of Family Guy
Stan gains a lot of weight in an episode of American Dad
Jake and Finn both gain a lot of weight in an episode of Adventure Time
... and Futurama
... and Aqua Teen Hunger Force
... and Metalocalypse
... and Sealab 2021
... and Southpark
... and this list could go on for several threadpages...
... and live-action shows like It's Always Sunny in Philadelphia...
... and hundreds of webcomics...
... and comic games like Sushi Cat or Fat Princess...
In fact I am confused about the entire premise of your challenge to this idea. This is a very well-established character modification with an enormous degree of precedence.
Notably in several examples I've mentioned pole dancing is also used in episodes, but only in the context of classlessness, self-destructive moves towards oblivion, and loneliness edging toward total despair (unlike Riot's Kat stripper)- preferred by only the worst characters and inevitably leading to horrific consequences. (ATHF Alien abduction of Carl the slob- Metalocalypse, the awful bachelor party of the brother of Pickles- ASIF, antiheroes pretending to be handicapped and being doomed to continue life in Philidelphia, etc.). In the context of female empowerment I agree with some writers in this thread that poledancing can be a cute and fun personal practice, but if one studies the animation more closely put into the game it doesn't seem as though Katarina is having fun at all, she seems to be joylessly working, which more closely follows the concerns by players writing about lionizing the sexual exploitation of poor people...
Is this really the rational that gets Riot going "wow... fat people in cartoons... better nerf irelia"? You can do better than this. (for the record, you can too Riot.)