While I agree that you can't reasonably implement a complex subject from first principles every time you want to talk about something, I think the core of the complaint was more in frequently calling something trivial when addressing a room full of students with varying ability.
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.