Hmm, I am in the same boat as Janet, and I think frump is in the eye of the beholder. "Round", to me, is not necessarily danger territory, as I view round diamond studs as timeless and always chic. There are a lot of stud earrings that I would consider understated but not frumpy.
Button earrings are touch and go, and it would really depend on the overall style of the woman wearing them. There are some people who can make the craziest things look good, so I would never rule anything out. However, I had a librarian when I was in grade school who looked and dressed like she was 65 when she was probably 30 years younger. She looked stereotypically like a librarian, with the eyeglasses on a chain, the big pearl button earrings and dowdy boucle skirt suits. Because of that association, I have difficulty seeing pearl button earrings as stylish (pearl studs are an entirely different animal, IMO).
Anyway, I had honestly never even considered that earrings could be considered frumpy prior to this thread. I just don't think that jewelry stands by itself in that manner, generally speaking. To me, it always either enhances or detracts from an outfit, but it never defines it. And I think that if someone looked well dressed and stylish in other aspects, I would be unlikely to see their jewelry as frumpy because I wouldn't separate it from the outfit or the person.