We lived with my MIL for a while, and she, too, had a habit of buying stuff we didn't need, and she, too, bought my child's first shoes.
TBH, it was her house, and if she wanted to fill it with tat for the kid then I let her - I just didn't use anything that I didn't need, and when we moved out, we left most of it at her house 'to use when we visit'.
Even the shoes, which upset me at the time a bit, has completely faded into no feeling by now - he's my son, 10 years later, I just don't care who bought him his first shoes (especially since we were absolutely skint at the time, so it was really exactly what we needed).
I know your situation is different, but I think OP, if you can find a way to just step back in your head with a 'this too shall pass' - use what's useful, ignore what isn't, and use your money to buy anything you actually want, rather than the basics which you can probably cover from her stash.
You do need to set some boundaries though, so the resent doesn't grow - maybe start a routine of retreating to your room now, so you can do it later with the baby when you need some peace.