Mabel has always made the Halloween costumes for both herself and Dipper. It’s just natural–Dipper loves the costumes she makes and she loves making them, it’s just assumed that’s the way things will go.
But the Halloween after they come home for Gravity Falls Mabel hems and haws about costumes, pointedly wondering aloud if she should make them this year. After what happened on Summerween she isn’t sure if Dipper wants to dress up, but she doesn’t want to do it alone either.
Finally she point blank asks Dipper if he wants to do Halloween costumes this year. He goes to his backpack and pulls out a notebook with something drawn on it and hands it to her. She looks it over and grins.
“Do you think it’s too much?” Dipper asks “We could do something simpler.”
“Not a chance!” Mabel cries as she runs to the kitchen to blend up some Mabel Juice for the long night ahead.
When the kids show up to school on Halloween, most people don’t know what to make of their outfits. Dipper’s right arm is made to look mechanical, with sticks strapped onto it. Mabel’s left arm has a glob of amber plastic running down it that culminates in a t-rex puppet at the end. Both of them have layers of cardboard over their clothes to look like the criss-crossing of logs. Mabel has an inverted triangle with a glowing LED circle on her stomach. Dipper has a yellow, lit-up triangle on his chest. Their legs are made to look robotic, with flattened pieces of cardboard on them that resemble billboards. Both of them have flat, ridged pieces of cardboard on their opposite arms so when they stand and high five they resemble a building with a peaked roof.
“Is it from an anime?” most people ask them.
“Yes.” Dipper says before Mabel can launch into a detailed explanation of the day the world almost ended. “It’s from an anime.”