Diminishing returns do come into effect, however they are proportional to the amount of additional frames. like another comment mentioned, going from 60 to 90 or 120 are big leaps that you absolutely will notice. Something you might not notice immediately is that some actions will just feel faster and more precise, even if theyre not particularly fast moving, like moving your cursor or scrolling a web page.

Personally after having a 120hz desktop i cant go back without it feeling slow and unpleasant.

My phone can also do 120hz but unfortunately it sucks the battery dry or i would use it there too.