what lesson about marriage does Austen convey when she writes about the deteriorating relationship between Lydia and Mr. Wickham?

I think her criticism of marriage for financial gain and superficial reasons are at play here. Ironically, precisely through their refusal to turn marriage into a business transaction, Elizabeth and Jane end up with husbands who are both very wealthy and perfectly suited to their characters. Lydia, with her delusions of money and instant romance, remains the most unhappy of them all.