This comic started as a vague idea that Grace would use a completely not at all gratuitous transformation to give herself some manner of advantage in speech class (this idea was the result of entirely pure thoughts that anyone would naturally approve of).
Then I tried to figure out how to actually make that a workable punchline that would prove just how not gratuitous an idea it was, and I somehow wound up wondering what Susan would have to say about it.
I found myself a bit surprised by what felt most natural for her, but it makes sense. Susan wants people to have the freedom to dress how they want. If it genuinely makes them happy, that's what she wants them to do. Telling Grace not to would remind her too much of existing dress codes for girls that are based on the assumption that boys wouldn't be able to handle girls dressing comfortably in hot weather, which is pretty insulting to absolutely everyone involved.
Granted, Grace is talking about actual transformation, but she's a shapeshifter. For her, growing a couple inches taller is like an alternative to wearing high heels.