I get the coniption but Nino revealing himself to Marinette in Illusion made sense.

Back in Queen Bee's time, there was a Miracle Box with other jewels that had to be kept secret. But Nino basically was under the impression that desperate times called for desperate measures and felt reveal he's Carapace to trusted ones for his Resistance to take off made sense at the time.

When Monarch has all the Miraculous, including his own, what's left to lose? Hell, he'd probably step down as Carapace given how he was commited to the Resistence.