Is it acceptable for a referee to suggest changing "theorem" into "proposition"? In mathematics, it is usual to call terminal results "theorems" and intermediate results "propositions" or even "lemmas" depending on importance and place in the overarching proof. Suppose that one is refereeing a paper where the authors have decided to call almost all their results "theorems", making a paper with a large number of "theorems" that even emeritus professors don't usually reach by the end of their career. (Such theorems include computation that could conceivably given to as end of year exams to master students. Not to diminish the importance of the paper, the actual theorems are good, but the 20 others are not theorems. There are more theorems than pages.)

Would it be acceptable and well-received to suggest toning it down? Or would it be overstepping and rude? This is not just a philosophical question: I truly believe that it makes the paper harder to reader, because it is difficult to separate the wheat from the chaff, so to say. A reader does not know what is important and what is not.
I think we should begin by acknowledging that it is the author's choice whether to label results Theorem, Lemma, Proposition, Remark, etc., so a referee should not insist on changing this as they should insist on the correction of an actual error or gap.


  Suppose that one is refereeing a paper where the authors have decided to call almost all their results "theorems", making a paper with a large number of "theorems" that even emeritus professors don't usually reach by the end of their career. 


Don't say that to the author, by the way.  It's snarky and not really helpful -- you seem to suggest that the author is somehow cheating their way to "too many theorems."  That's not a thing.

Referees can make stylistic suggestions, of course.  How much they should do this is quite a judgment call. I think a good referee should make stylistic suggestions when these suggestions impact the readability of the paper or affect its appraisal by readers.  Will it look a bit weird to many readers to have multiple theorems on every page?  Yes, I think so, and perhaps the author deserves to know.  However, if by having "too many theorems" the author makes it hard for the reader to see what are the important results of the paper -- and, as a statement about human cognition rather than mathematical achievement, a paper simply cannot have, say, 50 important results -- then that's a much bigger deal.  

If I were you, I would lead with the latter: explain that you had trouble isolating the important results of the paper because so many results are being presented in the same way, then suggest that changing some theorems to propositions or lemmas might be helpful in alleviating this.  (Then, if you like, say that having fewer results called theorems might make a better impression on the reader.)  I think this is putting things in the right order and does the best possible job at getting your concerns addressed.

But it is possible to address the main concern -- that the important results not get drowned out -- while still calling every result a "Theorem."  If the author pulls that off...okay.  Up to them, really.