D.B.
Well, you could start by telling your mom to put a sock in it! ;-)
It's nice that she wants to help but structure is important for a child - if you get a formula that works, you stick with it. You can get your mom to return it or you can accept it and donate it to the local food pantry where it will be much appreciated. Your mom can give you the coupons and let you pick what you want to use.
To some degree, you have to let the 2 moms work things out for themselves. You can't get in the middle, although you can express sympathy for one when the other is pushing too much. Your MIL may appreciate a sympathetic smile and a "What can I do?" shoulder-shrug when your mom is being pushy - she may not need you to do anything. You could say, "I think the flowers are fine, Mom. Can you help me with XYZ while she does the flowers?" If she's criticizing your MIL's cake, you can say something, or you can stay silent and let them handle it. If no one comments, your mother may notice the silence and figure out that no one has a comment. You can also say, "Look, everyone has their favorite recipes and there wouldn't be 40 million cookbooks if there was only one way to do things." You can ask your mom to make YOUR life easier by leaving your MIL alone.
If you think it won't backfire, you can also feed into your mom's feeling that she is more hip than the MIL, and just ask your mom to be more sympathetic to the MIL who can't do as much - "let MIL have her small pleasures, Mom, since you can get out more and be more active with the baby." That sort of thing. Your MIL is really old enough to be your mother's mother, so there should be no competition.
Your mother may be jealous of your affection for your MIL as well as the time your MIL spends with the baby. She's trying too hard. Has she always been this way?
You can try to spend special time with each of them but not together so that they each feel appreciated and loved.
As your child gets older, conflicts will get worse - soon you will want to do things like have holidays at YOUR house because it's too hard to haul the little one around or too disrupting for her to sleep elsewhere. Both moms will want a piece of you. You'll have to put your foot down, and you and your husband will have to establish that YOU are the parents of this baby and will make decisions. You can encourage BOTH moms to ask you what would be helpful, rather than to decide on their own and/or do it for you.
There may also be some resentment about your working FT if your mom was a SAHM - she may be trying to justify her own decision when you were younger by showing you that you can't do it alone if you are working. I don't know how you solve that - I just suggest it as a way of looking at her motivation.
Good luck - just work on setting boundaries NOW while you're figuring out all the aspects of being a new mom. Allow yourself the time and space to do this by encouraging your mom (and maybe your MIL) to let you figure some things out without a lot of advice.