In the subreddit dedicated to the animated series by Alex Hirsch, you / hitchhiker posted a photoshoot from the show, along with a caption that read, “Adorable detail from the show’s end credits: Mabel learns to love herself!” One of the polaroids showed a tree that Mabel often visited to carve her name and the name of her current lover inside the heart. The continuation showed that all pairs of hearts were crossed out, except for one. Mabel carved her name into the same heart twice, essentially saying that she was happy to be loved on her own.
This can be viewed in two different ways (this is a lie, but please accept it for simplicity). First, since Mabel is still a child, this can be interpreted as a child repeating the social constructs of romance. Considering Mabel is known to love stereotypical media for someone in her age group, this doesn’t go beyond plausibility. That being said, Gravity Falls does a pretty good job of adding additional meanings and character development, so it’s much more likely that it was intended to be about self-love and acceptance. Considering that Mabel is shown as a creative person and has been verbally longing for an epic summer romance, the thought of her going for this dramatic symbolism is entirely reasonable.
Going back, yes, this is probably a ploy to get viewers to digest the credits (which Den of geeks arguing about necessity), but it’s sweet and everyone could use a little extra joy.