The Flumph is pretty much irredeemable, but if I were to try my hand at it, I think I'd kind of model them after the ancient Yith of At the Mountains of Madness.
The normal flumph is essentially a sort of "caveman", the degenerate - yet still intelligent - remnants of an older Flumph society reduced to a sort of barbarism due to their loss of psionic abilities; perhaps hint at their downfall being orchestrated by one of the other abberant race such as mind flayers or aboleth. Flumphs might congregate in ancient ruins ripe with psionic power, moping and anguishing over what they have lost, or be completely oblivious to it. They don't have to be of low intelligence - they could be a mental match for a human, but they are but a shadow of the race they once were.
Monastic flumph are those flumph who still maintain their powerful psionic/magical abilities. They treat and care for regular flumph as we might treat someone who is mentally deficient (to monastic flumph, common flumphs would essentially be in the same category). Much fewer in number, they protect their brethren, seek to reestablish the grand society they once achieved and may even seek out psionic individuals to study or experiment upon in a vain hope to "fix" their brothers.