Okay, I believe Mabel would like to hold the middle ground, she would enjoy transforming him, but I doubt she wants him to look like an actual girl, that would put her off. So I guess she would mainly focus on a makeup and just ask him to wear some random one piece dress, those are pretty harmless.
Gideon is into humiliation, something skimpy and raunchy.
With Wendy I imagine it going like this: she sees some funny skirt, ask Dipper to try it, she looks at him and laughs, says something like "That kilt really fits you" or similar. I don't think she is evil enough to make him wear girly clothes to make fun of him. She somewhat respects him.
Dipper I would say is the hardest of all. He likes being seen as a man and absolutely despises anything that would make him seem girly and wimpy. But if it was absolutely necessary, he would have probably asked Pacifica, and she would have lended him one of her costumes. But if it was before her redemption, I would say he would have put a wig on himself, some low effort costume and called it a day.