Oh, moms! Always finding a way to guilt us into doing what they want, even if it's nonsensical.
Question: How, exactly, does she know the financial status of those kids? They could be very well in the same economic bracket as you, she just assumes they're poor because they shop at Goodwill! Which, uh, doesn't make much sense, because plenty of not-poor people shop there, too. Is your mom gonna set up a toll booth at the front of the store: Must Be This Poor To Enter?
Personally, if it's your money, I would just say to buy what you want. It's not her money, she's not earning it, so she has no say in how it gets spent. (Unless it IS her money, in which case ... she does, ha ha!) That said, I wouldn't go buy any in front of her, either; if she's not comfortable with it, no one says she has to know where the ponies came from, exactly.
Yes, kids enjoy toys, and I'm sure if you leave ponies for them, they will be happy! But kids aren't suddenly more 'deserving' of anything than adults. Do kids 'deserve' to have more fun than adults? Of course not, everyone deserves to have fun! And that includes ponies, I think. Do you have to buy ALL the ponies next time you go? Maybe not. Maybe every trip, you only vow to buy one pony, and leave the rest for the kids your mom is so very concerned about. (Who, BTW, may or may not actually be poor. And are not her kids, even!) Or, maybe for every G1 you buy, you vow to donate some G3s or G1 doubles! Or heck, even some G4s - I'm sure kids would love to get a hold of those for a lower price than in the store!
Long story short - your mom has no right to control the way you spend, especially not based on imaginary poor kids who, according to her logic, deserve ponies but not clothes.
And on top of all that - G1s might not necessarily be safe for kids who are going to lick/chew on them, since the plastic is SUPER old and starting to break down, in many cases! Is it safe for kids to eat plasticizers? I sure don't think so!