Is it really that hard to understand? At its simplest level, the NPC meme is an extension of "I'm a member of [insert outgroup here], look how dumb I sound" trolling.
SNL can pull off a loud and obnoxious Trump character, because they show they can work in many other registers. But SNL itself doesn't become indiscriminately loud and obnoxious throughout a whole episode in hopes that people will make some meta-level connection between general obnoxiousness and Trump. And for good reason; a tiresome character in contrast to non-tiresome characters can be good drama. A tiresome show is for most poeple just a tiresome show. There's a reason badfilm like The Room is very much a niche taste.