Yeah it's about stagnation and, in a way, acceptance. Relishing in the eternal rot. You will contract every disease imaginable and become a disgusting, foul abomination, but you will learn to love it, that's one of Nurgle's promises. I don't like Nurgle because all the body horror stuff is too icky for me but followers of Nurgle are some of the few people in Warhammer who usually seem like they're having a good time.