No, Frumpy is not a slur.
While "Frumpy" is not a slur it still is a derogatory or even insulting adjective.
The word "Frumpy" is a word that is used to describe someone, and almost always a woman or their clothing as being old fashioned, drab, unfashionable or even messy.
Although the word frumpy is an unkind and critical and often sexist judgement about a persons appearance, the word frumpy does not fit the definition of a slur.
The reason why the word frumpy does not fit the definition of a slur is because a slur is a deeply offensive and harmful term that is directed at a group of people that is based on their protected characteristics like race, religion, ethnicity, sexual orientation, or disability.
"Frumpy" is what simply describes a perceived lack of style or neatness.