Mutation dreams are a function of
- how heavily mutated you are (total mutations)
- what mutation category is most common in your character (not counting any starting traits)
Any mutation you acquire during play will affect dreaming, whether from radiation, yugg (does anyone actually encounter those?), oral mutagen, injected serum, rat-king aura, or scientist’s mouth-beam. Serum is necessary to breach a threshold but not to reach one.
Dreams will NOT change without a change in your mutations. If you go for a year with the same set of mutations, you’ll have the same dream-set that whole time, and will continue to have that same dream set until you remove or add mutations.