"Not bad, you made me use 10% of my power" -Frieren probably
Can't say I agree with the sentiment that hiding their power level is a mockery of magic. Dishonorable? Maybe. But a mockery of magic? It's just another technique same as everything else.
@Tersione Most likely the view of "magic is just another technique" is also anathema to mages in their world. "It's altering the basic building blocks of the world, and should be considered sacrosanct and with the gravitas it deserves! Magic is holy!" or some such mentality.
@Amariithynar According to Flam in the flashback, the three demons they faced revealed their power when they realized they were mages even when they were hiding beforehand. As she puts it "they all possess a senseless pride in their magic", so it seems more like a demon thing than a rule for all mages.
@Pyoro If that's all you've been able to glean, that's on you. I sure as shit am not in the mood to write a bunch of paragraphs explaining the real-life parallels considering the sheer density of them. Maybe haremshit fantasy is more your speed?