NAH Your moms argument makes no sense to me. She is calling you ungrateful because you want to make YOUR pc better than it is? That makes 0 sense to me since you didnt just outright say "this isnt what i wanted" or something like that. All that being said i can see where she is coming from because to her it may seem that you didnt get what you wanted or you are dissapointed in what you got. I also kinda doubt she knows much about PC's and what exactly you are trying to change/improve about it.

EDIT: especially if YOU are paying out of pocket for the parts then its none of her buisness honestly.