Yes it could hurt a frog if you place uth under tank, it will warm up a substrate and jt will destruct natural environment, frogs burrow not only to feel more safe but to cool themselves if want to, but more importantly a frog may burn itself on it. You dont't want any of that to happen. That is why Grif said to place it on a side. Keep in mind you will need a bigger sized heat pad if you want to heat up the tank.

I personally prefer infrared lamp(50w) +fogger +hydrotherm. This way you can get spot on humidity and temps at any given moment, it is expensive initial investment but it is so worth it