Same here - funny enough, I had 3 dogs last weeks and didn't seem to have that problem. Then I moved one dog into another household, then moved a Sim in, and now the two remaining dogs are constantly dirty. But in my game, I think it is linked with the fact that they seem to keep unlearning things:
I keep having the message that they learned how to go potty, but within the same day of receiving the message, the dogs keep needing to be told to go potty outside. The doors are unlocked and once asked to go potty, they go, but only after being told, not autonomously, then I have the message that they learned to go potty, then I get an uncomfortable moodlet as the dog need to be told to go potty again. It is the same with learning not to roll in puddles, only to do it again on the very same day, hence both dogs being constantly dirty.