This is the kind of question that is asked about a novel only if it is one worthy of a great deal of thought after its conclusion. To Kill a Mockingbird is certainly one of those books.
I'd like to think that, in a way, Mayella is better off after the death of her father. He was of little help to the family, and his unmentioned actions toward Mayella and the rest of the Ewell children will not occur again. Bob wasn't much of a father, so it wasn't a great loss to the family. The rest of the town probably breathed a sigh of relief to finally be rid of its most disgraceful citizen. As for the rest of the family, their situation could not have improved dramatically. Mayella may have received additional Federal funding following Bob's death, but it probably wouldn't have been enough to make the family life much better. The author never reveals if the Ewells had kin anywhere else, so Mayella may have had no one else to turn to for help. Employment was not a likely option for her, since she was now the sole adult, and there were other younger children to be looked after. More than likely, the Ewells would still inhabit their tattered little house on the bad side of town, and continue on as the "disgrace of Maycomb" for several more generations to come.
To me, this depends a great deal on what kind of person you are. Are you optimistic or pessimistic?
If you are an optimist, you might think that things will improve for the Ewells. With Bob Ewell dead, Mayella will be able to improve herself. Maybe she will clean herself up, go and get a job, and provide some sort of a better life for her siblings. Maybe the people of Maycomb will help her.
If you are a pessimist, you can say they're doomed. Mayella is 19 years old and uneducated. How is she ever going to get a decent job? She has no skills that we know of and she has all these kids she has to take care of. They will just continue to be the worst off people in Maycomb. In addition, she will not be able to get married because the white men will think that she was either raped by a black man or that she was coming on to him. Either way, she's in trouble.
It's likely Mayella and the children did not lead worse lives after the death of their father in To Kill a Mockingbird. The only thing we know for certain he did for them was to provide them with some meat from his illegal hunting expeditions. In this time, it's likely all of the older Ewell children could shoot, as evidenced by Walter Cunningham's experience with a rifle. The struggles would have continued, but the unmentionable behaviors would have stopped. We do know Mayella was able to save a little money from some source, so perhaps there was the possibility for more. In any case, the town's attitude toward them would undoubtedly have softened once the unpleasant and violent presence of their father was gone. We don't know what happens to them, but the fact that we care about their future says there is some hope for them.
I actually have often wondered about this. I agree with bullgatortail that the Ewell family would probably continue on as the town disgrace, at least for the present generation. As much as we feel for Mayella in coming clean with what really happened, I also agree with pohnpei397 that she has no real future, especially if she stays in Maycomb. The rest of the children are probably going to grow up to be similar personalities to their father, who was incredibly domineering. As sad as it is, I don't foresee a bright outlook for the Ewell children or Mayella.
In her novel, To Kill a Mockingbird, Harper Lee, as a resident of southern Alabama herself, does a realistic presentation of the majority of the characters. The "buckra" of the southern town, as represented by the Ewells, are, unfortunately, like Luster Sexton of Erskine Caldwell's Tobacco Road: No matter the lure of a life elsewhere, no matter how deprived their present life, they so often go nowhere. If they make any attempt, it is a futile one.