Absolutely regular corn starch can be used. The only concern about baby powder is what may be added to it, like fragrance etc. that could possibly be harmful to ingest. Good old grocery store corn starch is a staple in my cupboard for my bunny. And due to my fragrance sensitivity, I filled a powder puff jar in my bathroom with the plain cornstarch for my own powder. It’s nice!
Sorry she’s having pumpkin trouble. I also advocate a half bath in the sink with warm water and mild soap or shampoo, or nothing but a washcloth, to get it all good and clean. You’re experienced enough to know how to hold her so she isn’t submerged and doesn’t get her head and ears wet. Towel dry and back into a clean cage, maybe with a very low, cool hairdryer first, and she’ll finish the job. The corn starch will also help dry her. They are miserable from the yuck, and sometimes struggle with the bath, but obviously happy with the results. Bunnies want to be clean.