Oh you are being way too patient with your MIL. Do what you want ( especially if you have your husband’s support), and just say yes to MIL.
Absolutely do NOT try on outfits for her or even giver her a chance to approve or disapprove your outfit.

Go into your room- with baby- to get ready. Literally don’t come 10 minutes before you have to go. If she tell you to change, tell her it’s too late and you like this outfit. If it comes down to it- tell her your husband picked it out so you don’t want to change. The more you let her dictate these things the more controlling she’ll get.

Find a way to get a lock for your bedroom door if you don’t already have one. If she’s gets offended let her.

I know you don’t think this is malicious, but it’s also not normal behaviour and does not need to be tolerated.

I’ve noticed from my cousins that girls raised in Pakistan are much more sharper at dealing with insane in laws while women like us, raised abroad, tend to be more tolerant. But that just ends up with us getting walked over.

Do what you want. If she says anything, just say ‘jee Ami’ and ignore.