It all depends on how you think she would react. If you think she would act positive (unlikely) or neutral (possible) to the idea then go for it. Only you can judge that because as everyone else said, we do not know your mom.
If you think she would have a negative reaction, I would also suggest telling her another secret, but one that is a real secret you have so you do not have a chance of getting caught in a lie. A good secret I just thought of to tell her is to tell her you wet your pants on accident the other day, see what her reaction is on that. Just say you washed your clothes before anyone noticed, but you wanted to tell her because you were worried about it. If she has a negative reaction to that, just drop it and do not tell her about the diapers. If she has a neutral reaction to it, I would also suggest dropping it. Although if she has a positive reaction by saying that "its okay, accidents happen, if it happens again tell me and we will help work it out." Of she says something similar to that I would say you might be in the clear to tell her about the diapers. Just say that you got some diapers and started wearing them. If she flips out, say its cause of the accidents, if she doesn't flip out tell her the truth and tell her it is because you enjoy them.
Good luck and use the best judgement you possibly can!