Mrs Bennet's income is not large enough to compensate for the small size of Mr Bennet's fortune.

"Could but ill" means "couldn't really." "Deficiency" means "the amount he needed, but didn't have." Mr. Bennet needed a lot more income. His wife had a little extra income. Let's say he needed £5,000, and his wife had £500 extra that she could have given to him. Could £500 extra make up for a deficiency of £5000? No. The short answer is "it couldn't." But that's too strong. It's too definite. She had something. It wasn't enough. It wasn't close to being enough. Instead of saying "it couldn't," Austen writes "it could but ill." It could--but only very poorly. The English used in Jane Austen's novels is difficult English. Native speakers find it hard to read. The sentences are long. The grammar is complicated. Literary English went through a revolution in the 1920's, 1930's, and 1940's. Novelists write in simpler English nowadays.