Trivial is, I suspect, best expressed in terms of inferential distance that the student has to cover. Education naturally works on the borders of what people know: too far and showing people something is incomprehensible to them, too close and they're learning nothing they couldn't have found themselves simply by looking at your powerpoint stack.
I suppose, to develop that line of reasoning, it might go something like this:
Given that you're operating on the edge of people's concept space to be teaching them something worthwhile, if you're saying that something's trivial (i.e. that they should find it trivial) a lot, you're either:
A) Wasting people's time, (it's too close to their known concepts)
or
B) Confusing people, (it's too far from their known concepts)
If you're in the goldilocks zone for learning, it shouldn't be trivial. Might it rely on things that are trivial to them? Sure. But I don't see any value in mentioning that they're such, and if you're dealing with varying ability it's worth keeping in mind that some of the things you think are trivial aren't going to be to everyone.
Basically, my question to you, would be: What value is added by calling something trivial to justify the harm to those who don't find it such?
You could after all just not mention that the thing is trivial, perhaps more students would have the courage to ask you about it if they don't understand it that way.
I also don't think a university course has to hold every students hand. For example if you offer Advanced Calculus or whatever, it is fair to expect students to know that 2+2=4. If some step is too far from a students known concepts, they can either invest extra time trying to catch up (Google is your friend), or they can switch courses.
You could also debate which approach is better for learning maths. In my time, there were comparatively thin books about calculus and comparatively thick books about calculus. The latter spelled everything out. I hated them - too many words really made it hard to focus. I personally much preferred the dense books that left more thinking to the reader.
Maybe for some students the thick books are better, but I don't think a teacher has to please everybody. Students should have the opportunity to switch to another teacher or subject if they can't cope with the current teacher.
For example a pathological problem doesn't mean you'll get a nasty MRSA infection from working on that problem. At least not directly...
The prerequisites may have been taken 6-12 months earlier. Without constant use, a lot of the concepts that were learned in the prerequisites will be lost over that time period. Many classes will indeed start with the first lesson or two as a revision of the prerequisites for this reason. It possibly annoys those who have a firm grasp, but not everybody learns mathematics (or language, or anything else) the same way.
