Are the merch mimics like, metaphors for how capitalist brands have sort of taken on a mind of their own and how mascots have replaced gods and demons in the modern world?
Not precisely? with how merch mimics form, they're moreso metaphors for how OCs and sonas get made via inspiration from other characters and things in an endless cycle, and how characters can live on beyond canon via readers re-telling stories and reinterpreting things through their own individual lenses. The only mimic so far that matches what you suggested is one Angel oc I made, Hambaga-Desu, having clearly taken some inspiration from Ronald McDonald, but that's not the only way angels can exist.
I should clarify and concede that some mimics are in fact demons and yokai of old that changed form to keep up with the times, but plenty more are wholly new beings who don't fit snugly into the role of being the new gods and demons of the world. Jack for example is more or less the equivalent of a lost media character that never got their chance to properly exist, yet somehow barges through into existence anyway, like when fans find scraps of something unknown and make their own thing out of it, like when goncharov became a thing. There's more nuanced differences of course but that's kinda the idea of how mimics spring up.
to recap, Merch Mimics are sentient ocs and fursonas that just pop into reality with no singular particular owner. Furthermore, they avoid trying to match pre-existing characters, because if one tried to be Mickey Mouse, they'd likely just get obliterated by an angel, or be too obviously a copy with personality quirks too unique to themself, e.g. a Mickey Mouse that acts too much like just some guy who is a fan of Mickey and acts OOC.