I think JA uses it as an understatement to describe Darcy's feelings for Elizabeth, as she so often does. IMHO, she means with 'a tolerable powerful feeling' something which is in actual fact more powerful than the expression seems to imply.

IMO, to be called one of the silliest girls in England is more disconcerting than being snubbed at a ball. Besides that, Mary has a thick skin, and doesn't possess a functioning social radar.