Yeah, i have mixed feelings about this episode as well.
Particularly that noone bothered pointing out that the rest of Gravity Falls was beeing torn apart by Bill and his minions, and that Mabel prioritized being happy over that. Escpecially considering how obsessed she was with proving that she was a good un-selfish person in "The Last Mabelcorn".

I’m inclined to agree with this.
I mean yes, she is only 12 and doesn’t want to grow up (which is very understandable and relateable), and she missed the whole demons-coming-through-the-rift-and-destroying-the-town part. But there was this moment when a portal to the real world opened and they could totally see how fucked up the reality was, and not even once we saw her thinking “Oh no! What happened to all those people? What about the grunkles, what about my friends?” Also, she wants her brother to be with her (which is great), but if not, a dreamt-up “cool” substitute would suffice – seriously, WTF Mabel?