For the record, Roland's whole deal in his early life was to try and fix Mousillon. Kill the monsters, consecrate the lands, dig new wells, bring in the Cult of Shallya, bring peace and purity through the Lady, etc. He and his older brother Maldred tried to do so.
Mousillon was definitely past its early glory days, but was still considered a wealthy if much smaller Dukedom than it used to be, and Maldred a known shrewd and capable commander and knight by the time that Royach Jules the Just died. For DoDA purposes, it's framed that Maldred became known as such and was made the Duke by the King for his deeds after the previous Dukes kept dying out and failing to hold back the steadily worsening conditions of Mousillon, while Roland was a Knight of the Realm.
But then came Malfleur, and the Affair of the False Grail, and then things really went to crap.
It's only really after that, where no Duke was appointed to replace Maldred, that the Dukedom started to decay more and more until it became the ultra plague-ridden undead infested hyper-mutated land of darkness that it was in 2522 IC with the Barony of the Damned being its newer name, plus the Black Knight, and other such issues of a land that became well-favored by Nurgle.
At the moment, in DoDA, it's already started sliding that way, with the Cordon Sanitaire put in place, but it's still missing most of two centuries of sliding into the muck. It's still definitely the 'worst off' of the Dukedoms, but that difference is far less of a gulf than it eventually would be in canon. It only slid into an impoverished awful place post-AotFG, and had managed to get pushed back into a wealthy position beforehand by the effort of the Duke and his knights.
That's the worst part. It was working. They were doing it. And then after Jules the Just died, Maldred thought that becoming King would be the thing to actually fully push Mousillon back into a fully glorious status - and he did what he did to try and get it.
Roland tried to stop it, stop him, failed, and the Fay Enchantress banished him from Bretonnia, and now he seeks the Grail himself, but for true. Not the lie that Maldred told and sold, but the truth. And perhaps, if he succeeds, not all of the blood of Mousillon is accursed.
If.