I am a mom and a sm. I've been a sm for 6 years now (officially, that is, we were together for two years prior).
IMO, it's good to hear bm's view of things. You may not agree, but it's better that she voices it, or writes it down for you, than to hear secondhand from the kids or thirdhand from dh or someone else. You may decide you can tone down some things, or she may realize she's being irrational in saying you're "trying to be Mom" when you cook their dinner.
Are you custodial? IF so, it may be just bm lashing out at the resentment she feels because of that, and it's nothing at all to do with you. You may be just an easy target. Us sm's are, y'know. :P
I am a maternal person. I do what needs to be done to ensure their care and protection while they're under my roof. I do, however, have boundaries. I often get/got the awkward questions. My first response is, have you asked your mom (or your dad, if appropriate) about that? Sometimes they havent', to which I'll tell them to do so first, then ask me again if they need to. I just try to figure out how I would react if I were bm, and my kids' sm did x or said y.
If I were "trying to be Mom", I'd have them call me Mom. I'd probably PAS them against their mom. I'd make sure I was listed in the Mother spot under their contact info at school, etc. I wouldn't inform her of parent/teacher conferences, dr's appts, soccer games, dance recitals, etc. I may not do all these things, but even doing only a couple of them is intrusive on the Mom role that this other woman has a right to (assuming she hasn't forfeited it legally or otherwise). If our bm came to a meeting with a list of infractions I'd committed that read like this, I'd deserve what I got. If the list reads like a to-do list for the day - buy groceries, check homework, prepare bath, do laundry - then perhaps bm needs to give her head a shake and decide if she'd rather her kids live with a woman like that or a woman who doesn't give a rat's arse about the kids.