Oh the Santa Claus suit. Has there ever been such a beloved, iconic piece of clothing, besides Elvis's amazing white jumpsuit?
Our good old buddy the Grinch knows that the Santa Claus suit represents goodness, generosity, joy, and most of all, Christmas. And so, of course, he dons it while carrying out his plan to wreck the holiday entirely, proving that the original hipster was not, in fact, that college student in skinny jeans with a handlebar mustache. The original hipster—when it comes to dressing as ironically as possible—was in fact, the Grinch.
Just like the hipster who starts out with wacky facial hair (that proves some point unbeknownst to everyone else) and then slowly becomes inseparable from the aforementioned mustache, the Grinch eventually succumbs to the Santa Claus suit.
In the end, what the Grinch originally donned as a disguise (something that didn't represent him at all) becomes something that he wears because it fits him like a glove. After all, who brings back the gifts to all the boys and girls of Who-ville in his sled at the end? He literally has a Santy-Claus moment.