Specifically what does the speaker in the prologue of she stoops to conquer say about the sentimental comedy?

At the time of She Stoops to Conquer, popular theatre comedy was separated into what was commonly termed "sentimental comedy" and "laughing comedy." The former was concerned with bourgeois (middle-class) morality and with praising virtue. The latter, which dated back to the Greeks and Romans and through Shakespeare, was more willing to engage in “low” humor for the sake of mocking vice.

Woodward suggests that a certain class of actor (and by extension, then, audience and writer) were dying out as sentimental comedy became more popular. So Goldsmith's play has an extra purpose: it must rejuvenate the joy taken in “laughing comedy,” which could be willing to be more stupid, to dramatize base characters and characteristics, and to mock even the characters who profess to be moral.