Despite sounding like Dr. Seuss characters, these two ladies are bad to the bone. They're Mrs. Sinclair's oily sidekicks who don't seem to have a problem throwing Clarissa in the slammer. If you need any convincing that they're the worst, look no further than Clarissa herself: she calls them downright "vile" (333.31).
Why all the hard feelings toward our girl Clarissa? Lovelace tells us that the girls used to be somewhat decent before he made them his conquests. It's a slippery slope from Lovelace's girlfriend to a house of prostitution, ladies.
While Sally and Polly don't seem to be particularly nasty toward Clarissa at first, they can't help but resent that she manages to hold out against Lovelace. After all, misery loves company.