How does the following line from Lady Bracknell contribute to the humour of the scene in The Importance of Being Earnest?
'Never speak disrespectfully of society, Algernon. Only people who can’t get into it do that.'
This scene of The Importance of Being Earnest happens in Act III, after Lady Bracknell has a sudden change of mind when Jack "casually" points out that the net worth of Cecily, whom Lady Bracknell had shut minutes before.
Knowing the amount of money that Cecily owns,
A hundred and thirty thousand pounds! And in the Funds!
Lady Bracknell suddenly finds positive traits in Cecily, and even concedes that Cecily may have a much better potential in society than she had previously told her.
This is the moment when Algernon may have possibly said the first authentic comment regarding his feelings for another human being:
Cecily is the sweetest, dearest, prettiest girl in the whole world. And I don't care twopence about social possibilities.
Notice that Wilde could have used that comment to expand upon the potential good qualities of Algernon; he could have even made this into a truly romantic moment. Yet, true to his commitment to triviality, Wilde interrupts this solemn moment with Lady Bracknell's snobbiest comment made in the play:
Never speak disrespectfully of society, Algernon. Only people who can’t get into it do that!
The connection to Jack Worthing, and particularly to Cecily Cardew's money, has solidified Lady Bracknell's self-importance as far as being a member of the high society.
Hence, asking Algernon to not speak disrespectfully of society is basically saying that those who are less fortunate speak ill of fortunate people out of envy. Since they are not officially very much part of the high society in terms of rank AND money, she has the right to say as much. It is in the way that Wilde juxtaposes her comment in relation with Algernon's first sign of sincerity what brings the humor out of all this.