Is there anything Mathilde could have done to make herself happy in her initial situation? Could her husband have done anything more?

The only thing Mathilde could have done to make herself happy would have been to accept and be grateful for the lifestyle she was provided with, or to have gone and and taken on her own job to better help her afford the things she wanted. Sadly, Mathilde was not fueled by ambition, but rather, a sense of entitlement.

In my opinion, M. Loisel went to work everyday and provided his wife with whatever she "needed". Thus, it is my opinion that he didn't "need" to do anything more.