I totally understand how you feel, both my parents are a bit like this. I remember once dad getting out of the car, and between the driveway and entering the house, he had found half a dozen things to criticize: 'that driveway needs fixing; your gutterings filthy, that bin needs a scrub"! Then I'd say "morning dad" and he'd walk in, touch the radiator, and say it was either too high, too low, depending on what setting it was on. I once got home from shopping after leaving mum with the kids when they were small for an hour, and found that she'd rearranged the furniture in the living room "much better this way, love" she said. It then got worse and worse, with her having a go at my housework - nothing was ironed well enough, cleaned properly, tidy enough.
So one day I thought sod it, had enough. When I next went round hers I rearranged her lounge whilst she was making the dinner. She went bloody mad. I think it got through to her how I felt!!
I don't think they realise how busy we are with kids and stuff. My parents are both retired, my mum has a busy life, but has all the time in the world to keep a tidy home - there's no teenagers messing it up, just the two of them. Mine have forgotten how busy it is in house full of kids. I'm on my own, and I work full time, so some things have to slide...I am happy with it, so tough if they aren't. I still get tut tuts from her when she visits, but I totally ignore them. It's my home, it isn't filthy, but it wouldn't impress Kim and Aggie either! So what, keep telling yourself that you're ok with it, and just ignore the criticisms, and tell mum that you appreciate her help, but certainly don't expect it.
Good luck