That information is even more useful going downhill in mountain condition. For example, going down from Sequoia Park (30 minutes trip downhill pretty steep), is it safe to gently press the brake pedal all the way down, not saturating the blue regen monitor? Or should I worry about brake temperature and use manual shift for engine brake??
I haven't actually tried this with the Niro, but with other cars I've driven, a crude technique for discovering brake temperature is to pull over, get out of the car, and touch the metal part of the wheels (not the hub caps) to see how warm they are (if you only try this with one wheel, be sure that it's a front wheel since they usually do more of the mechanical braking than the rear wheels do). If you attempt this, you should initially regard the wheels as potentially hot enough to burn your finger: think of how you would test the temperature of a hot iron and maybe moisten your finger and only touch for a fraction of a second at first, until you are certain that the wheel is not burning hot.
When I've done this in the past, the reason for doing it was to discover if I might have one brake that was dragging and heating up more than the others. That's something that's more of a concern once the car gets older though.
In theory, the ideal discovery is that the wheels will feel like ambient temp. In reality though, they might be slightly warmer than ambient, because the rubber tires might also add some heat to the wheel (tires tend to heat up when you are driving by a small amount). If the wheel feels hotter than the rubber tire, then it's likely that you've recently been using your mechanical brakes in addition to your regen brakes, and the surplus heat you feel is an indication of energy lost to heat, rather than being recaptured by regen. If the wheel really does feel burning hot, then in addition to not recovering energy via regen, you might also be overheating your mechanical brakes, which is something that is easy to do on a long mountain downgrade in a conventional car if you rely on just the brake and not the regen or engine braking.