Mabel absolutely oozes Ne. She’s pretty much the culmination of the show’s absurd wit (where her brother represents the show’s serious side and mystery elements). She has a different sweater every episode which, according to the creator, she knits herself between episodes. On a whim she falls in love with a pig being offered as a carnival prize, names him Waddles, and knits sweaters for him.
An entire episode is spent tracking an insane dead president, National Treasure style, and they only make headway in the investigation because she has the exact same bubbly wackiness the president did (folding maps into hats, sitting in chairs upside down, pretending a statue is picking her nose…)
All of this whimsical Ne-Exploration is guided by her Fi. While primarily guided by her curiosity and sense of wonder, each decision is weighed according to whether or not she wants to do it, and the kind of person it would make her. Mabel is outraged at the suggestion that Waddles be used as food and demands he be treated as a family member, and has a tendency to anthropomorphize other animals and inanimate objects. In a later episode, when accused by a unicorn of being a “bad” person, she’s devastated and obsessively performs good deeds to counteract this blemish on her self-image.
She laments at one point that she thought she was “being charming” when another character calls her annoying and immature (a nod to her Delta-tastic separation from her perception of herself and how others perceive her). Mabel wishes for acceptance and true love, but never comes to the conclusion that her “normal” mode is not acceptable. She breaks the mold without being confrontational, content to just be Mabel with her circle of weird best buddies.
Mabel is very outspoken, and Kristen Schaal’s Te carries over in her characteristic nasal shouting. Mabel’s attempts at friendship and romance often consist of incredibly blunt and socially clumsy advances, though with no shortage of enthusiasm. She comes on very strongly, and often shows affection for her brother by picking on him or physically shoving him (in one episode we get a little insight that she treats him this way because his intelligence makes her feel insecure and un-special). Despite her lack of nuance in the social sphere, she manages to win people over (audience included!) with her confidence and ability to be herself unapologetically.
Mabel’s Te is very much in force, but it’s in conflict to her secondary Fi. She’s put in charge of the Mystery Shack for a couple days and proves to be a terrible businessperson, losing money because she’s always apologizing, giving things away, and being too lenient on her employees. Later in that episode she cracks down and channels her inner TeSi to fix the mess she’s made, but the idea of acting that way all the time scares her.
Mabel’s Si is expressed in a typical polar fashion. On a day-to-day basis she flutters from thing to thing at her whim, lacking an Si anchor to steady her and motivate consistency. Its healthy expression shows up in small ways, like her family values and nostalgia for things she didn’t actually experience, like terrible 80’s cartoons. Mabel’s Si really rears its unhealthy head at the end of the show as she’s confronted with leaving Gravity Falls at the end of the summer. The once unfettered Mabel becomes consumed with fear of the unknown and loss of the familiar, making irrational and dangerous decisions in an attempt to preserve normalcy.