Not him, but with the way she’s written, it’s really not.
Like when she leaves Dipper to try woo her crush of the week after promising to help him, and only chooses him over the puppet show at the last second.
Or “The Love God” where it starts out with her feeing compassion and empathy for someone else, however undeserving, but then quickly becomes a vanity project for her to apply her self-proclaimed matchmaking mastery. And she not only forcibly tampered with the minds of two individuals because she couldn’t bear to face the fact that maybe she isn’t so great at it after all, but also refused to reverse said brain control, simply because the consequences of doing so would have made her uncomfortable.
Or “The Last Mabelcorn” where, when she is told she isn’t perfectly Pure by the Unicorn and that she is flawed, an assessment which is actually true by the way. A good person would have at least considered that it’s words might have some merit, and looked upon themselves and their past actions in introspection, and realize that they are fully capable of being in the wrong and gain some humility along the way. Mabel however, rather than even consider that she could ever be wrong, instead embarks on a series of “good deeds” and penitence, believing she can buy the unicorn with Brownie points.
But of course, we can’t forget the most important example of them all: Mabel-land. Everything up to this point could simply be brushed off as OOC writing, but when Mabel ideal’s world was shown to be a sugar coated dreamscape where she was sovereign, celebrity, and saint, where her ideal companions were souless sycophants who bowed and scraped to her every whim, where her ideal brother was just a mindless walking trope, who acted nothing like her real brother, the one she couldn't bear to live without, and whose only purpose was to support her, it proved beyond a shadow of a doubt that Mabel was a selfish, narcissistic little girl.