I secretly recharged the mobile phone bill of the girl I had a crush on. Am I stupid?

Yes.

You have a crush on her, but she doesn’t know. Even if you recharge her phone bill, he won't know. Maybe she would think: What kind of scapegoat is so heartless that the phone bill is charged to me? Or think this way: Could it be Xiao Wang who made it for me? It turns out he likes me too. Note, you are not Xiao Wang, you are Xiao Li. In short, what you expect is that she will find out that you recharged her phone bill and that you like her. This possibility is too low. Instead of taking the money to recharge the phone bill, buy a bouquet of flowers and give it to him directly.