This quote refers to Marianne's attitude which is completely dissonant to that of her contemporaries in Regency England. She is a woman who is outspoken, opinionated, challenging, stubborn, and dead serious about what she expects of herself. She has all the qualities that maybe are expected of a gentleman, instead of a lady.
To her peers, she is trouble. A woman is supposed to be quiet, undisturbed, pleasing, eternally dependent, and subservient. She is everything but these things that are characteristics that mothers should have taught her daughters.
Therefore, being a sermon on the dangers of ill-regulated sensibility is the same as saying that Marianne is the epitome of imprudence, lack of class, lack of manners, and lack of good behavior according to the outdated expectations bestowed upon women in Regency England.