@moofolk
Honestly. mil's get a bad press, but they feel awkward and weird too. I find it helps to just assume good intentions and respond in that way.
I don't think it's 'good intentions' for a mother-in-law to start cleaning her son and daughter-in-laws house, without being asked or prompted. It's massively inappropriate and rude and presumptuous. And as I said, it's basically saying 'your house is a shit-tip, and you are clearly not capable of looking after your home and family.'
It's not a 'nice gesture' and it's not altruistic, it's bloody rude, and if my MIL had ever done it, I would have told her to get tae fuck.
Fortunately, she has never been so rude and obnoxious as to come and clean my fucking house, without being asked, like I am not capable of doing it myself! 
Oh, and if mother-in-laws get 'bad press,' you have to ask yourself why! They don't get bad press for no reason. Some are lovely - mine is, but some are not so sweet. Some are basically a nightmare, and some are just borderline...
Not awful, but passive-aggressive, micro managers, judgy, feel like they're superior to the daughter-in-law, do NOT like it that some other woman has taken their precious boy (and he loves said new woman more!) and just generally making life a bit uncomfortable for the daughter-in-law.
Let's not pretend that some mother-in-laws are not like this, because they are. I have seen it with quite a number of women I know.
Interestingly, it's only ever the mothers of sons who are the awkward/not-so-pleasant ones. Very rarely is it mothers of daughters.. Interesting........