I'm sorry it turned out this way for you. I had a very similar experience and it eventually lead to me moving out of my parents house because I personally couldn't stand the criticism anymore.
Is she right? In my opinion, mostly no. You spending your money on something you enjoy is your choice and you should be able to spend it on whatever you want. Also, sure it costs a bit of money, but at least you aren't spending that money on illegal drugs, or anti-depresants because of self hate for trying to stop being an Adult Baby.
I think the hardest thing people can't understand, is that we really don't mind the feeling of having a wet diaper on us, and actually, we like it. She might be of the impression that "[son/daughter] is mentally addicted to diapers for some type of security reason and doesn't feel like he can stop it." Rather than the truth which is "[son/daughter] likes to wear diapers and it has been his dream since forever, and now is finally able to express himself and it helps him feel better."
Something that might come across better than "I can't quit it," is, "I don't want to quit it because I like it, and even if I tried to quit it, I would be horribly sad to destroy this part of my life."
Your mom may never understand anyway, but stand up for yourself. The reason you cant quit, is because you probably don't want to, because you probably like it.
The part where you'r mom is right, is that it is money that could be spent towards taking care of your own physical needs. However, everybody deserves to have a little bit of play money to make life worth living, but your mom might not look at it like that. There are a few things that I would say about this.
Not sure what you think about cloth diapers, but I started to look into it heavily when I feared I might not have the finances to support my diaper habit. You might consider investing in just a few, so you can know that you will always have a reusable diaper to wear, this is for self comfort, to know that you will always have a way to wear. Your mom might not be happy to see you investing more money though, so you might want to keep that on the down low.
Second, try and keep it on the down low the best you can. Look into getting your diaper stuff sent somewhere else. You can send to UPS or Fedex with the "Hold For Pickup" program, where you pay $5 to pick up mail that is sent to them, or you can get a PO box. If you are especially bold and not worried about discovery too much, and you know your mailing is descrete, you could send mail to an office you work at or something, but that is probably not smart. Main thing though, find a way to send to somewhere else.
As part of the last, find a way to keep it even more discrete than it was before, find a way to make your mom think that it is starting to wear off and go away, especially if she wont buy into the fact that you don't want it to go away. You eventually won't have to worry about it anymore once you move out, but having your mom react the way she did is just super annoying and downright depressing.
Overall, I don't think she was in the right by reacting that way, it is super rude, and very closed minded.
I think in a (edit) right circumstance it could be worth saying something to the effect of, "I don't expect you to understand why I like diapers, but I would have hoped that you could respect my ability to know what makes me happy, and respect my ability to choose." Then if you really want to go this direction, which you might not, you may consider something like this, "Mom, if you don't want to spend money on my meals, and you would rather see me spend my personal recreational funds, which isn't much, on buying my own food, then I'll do it. I didn't feel like it was wrong for me to spend a bit of my money on something that I enjoyed because people need to have a little bit of things they enjoy in their lives sometimes, but If you feel like I should be spending all of my money on necessities and that would make you feel less like you are supporting my habits, which I never thought you were, then I am prepared to do that. I had always appreciated your help by giving me a home and food and getting me clothes, and I never was trying to take that for granted by buying diapers, I was only intending to spend money that could have otherwise been use on going to see a movie or something."
A different, less verbal approach that just came to mind, You could instead just start going to Ross or something and buy yourself some new close fairly frequently, maybe she will settle down when she sees you are buying clothes and looking fancy, it is a good distraction, even if you are also buying padding, which hopefully you can make her less aware of.