Hi @Rangervegas Welcome to the forum.
Depending on your sims traits they can either like or dislike the mud bath. It seems neat sims do not like it. Apart from that I think the negative moodlet is chance based per sim, a bit like how some sims like fruitcake and others don't.
Ok it seems strange but perhaps not so strange that a trait would keep a sim from getting a beneficial moodlet from Spa day especially considering how long lasting and unique that mood is. I tried it with another sim in the same household and get the same result but I believe both have the "Neat" trait. I will have to try removing that and see if it changes. I would think though the programmers would have been better served to allow for both moodlets on "neat" sims, since the negative one is relatively short lived and can be expunged fairly quickly with some type of cleaning actions after the mud bath.
I suppose this is another one of those things like Auras straight up not working or not working correctly when you are using cheats and have locked the needs decay of sims. I found out on my own about that one and it seems alot of people are confused about it since there are other bugs with the Auras where some items dont give any aura, or an incorrect amount or something.
In any case I will try removing the "Neat" trait first and see what and how much of an effect that has.
Thanks for the idea.