All good points.  It's subtle because she's always alongside the main group as a friend, but Muffy really is the main villain of the series.  Though she's not necessarily a direct adversary, she usually passively represents the wrong message or some immoral position that the "good guy" characters must avoid to successfully deliver a good, wholesome message to the audience.  Her position of wealth, vanity and self-interest is a lure and real-life temptation that the young audience is led to see through.

In that regard, she's a bit of a Satan figure.  Her extravagant wealth, value of the material over the spiritual, family tradition of deception, and frequently-changing hairstyles and outfits; a unique shape-shifting ability that no other character has shown to such an extent; all make her fit the symbolic role even more so.

And by virtue of her villainy, here Muffy has successfully managed to hijack poor George's nosebleed thread for herself.