Lady Bracknell is pleased with Cecily's appearance, but only in the sense that she thinks it will make an impression on society ("distinct social possibilities"). Her listing of lack of principle and lack of profile as the present day's two "weak points" is ironic to the reader, given that one of these is an essential component of character and the other is completely superficial.
Algernon objects to this because he considers Cecily beautiful in her own right, and what society thinks about her appearance is unimportant. Lady Bracknell responds that by disrespecting the opinion of society, he is only demonstrating that he's outside of it - in essence, she's calling him jealous. Only people who aren't fashionable ("can't get into it") reject the ideals of fashionable people.