This chapter is by Prez Cipher, who knows how each word ahead feels and just wants to give Mabes a hug.
It has been stated, and confirmed in "Little Dipper," that Mabel is jelly of Dipper because he's academically better at her and wins at almost everything. Mabel's only real ground on him is the fact that she's five minutes older and a milimeter taller. Plus, she's Grunkle Stan's apparent favorite, so she's got that going for her.
But still. What if everything she does is a cry for attention?
As far as we know, Dipper is the only twin that had to do the Lamby Lamby dance when they were younger. This may be hinting that their mother thought that Dipper was cuter than Mabel. The only thing we have to know as far as Mabel's relationship with her parents is the fact that she received her floppy disk nightshirt from their father. Perhaps Mr. and Mrs. Pines neglect Mabel and give Dippingsauce all the attention.
Everything Mabel does is needlessly silly and over-the-top. We all know this and love it. Her bright and colorful sweaters, her jokes and craziness. She was ready to throw them away when Pacifica Northwest made it worse in "Irrational Treasure."
And let's not forget that one somewhat forced quote. "Nacho earrings! I'm hilarious!"
Self esteem issues?