Frumpy is clothes that don't suit the wearer, not any type of clothes.
It's usually a combination of wrong colour and wrong style, badly put together. Decent hair and a smile both help to overcome frumpiness whilst a sad day and bad hair are no one's friend.
Some people look fabulous in a floral dress and flat shoes, others look awful. Personally, I look pallid and dull in beige which is supposed to be oh so cool.
Some people look edgy and amazing in clashing colours and patterns, others look like escapees from an asylum.
Some people look incredible in black. Most white british people don't.
Some people look awesome in skinny jeans, boots and whatever else is deemed to be so last season. Get over it.
Some people look stunning in the latest styles. Others look like a joke who got dressed in the dark.
Not frumpy is being authentic and wearing what suits you. Use a mirror, not a rule book.