Are 1-Ups any good as rewards in Super Mario Maker 2?

It depends.

If you have a somewhat challenging level it might end up in endless mode. There 1-Ups are at least a little bit of value, since you can collect 3 per level to add to your total live counter. Especially for every mode past easy it is helpful to have your live counter stocked up for stages that require a few more tries.

In normal mode, where you can choose levels as you wish 1-ups are really not worth your time, as long as you did not set the win condition to collecting enough.

It may be better to reward your players with a mushroom or a fire flower here, since those are more versatile usable if you value normal play over endless modes.

But I personally would refrain from giving the player a movement improving item (like propeller mushroom, cape or tanookie leaf), since that is usually a way to break a level and get through it really fast without really admiring it.

Another way to reward a player for finding a secret area might be, that you set a win condition. If the player has to get a set amount of a special Item and he finds one or two of them in your secret area they don't have to look for them later on. That does work pretty good with 1-Up Mushrooms.