"Frump" as a noun is not commonly used (at least not in the US). I don't think I've ever used it. However, "frumpy" as an adjective captures the meaning of мымра very well because it is funny in that it depicts an unattractive woman creature. I often say that I am "in a frumpy mood" when I don't do my hair or wear bad clothes. It's not terribly offensive if said with a grin.