It's actually a bit worse than you think. Your MIL has clearly done everything for your DH in the past and her behaviour is just a continuation to this. She clearly doesn't believe you are good enough at the 'home care' for her darling son. That's effectively what's happening, she is seeking to usurp you in the good home maker role. It's creepy. But that's at the heart of it and she'll be the first one to make any comment or judgement on what is going on in your lives, in particular yours. The obvious criticism over the plant and fairy lights is her way of gaining power and status over you in your DH's eyes, by undermining your competency, and that's what she will increasingly do, go round judging everything, tutting at your laziness and ensuring everything is in accordance with how she expects it to be for her son, the extension of her home, which is your home.
I feel for you, a tricky situation to deal with. If I were you, I'd invite her over for a coffee and a chat, explain that you need some space and privacy and for her to not turn up without being invited or expected. Put it out there in the open, behave as if your opinion matters. This is the gentle and kind request stage, that can't be denied. Don't bother with silly tactics, if she's that controlling, I doubt you'll be able to move home. That's too drastic. Talk to her kindly and gently to explain that her uninvited appearances are not appropriate behaviour and you require your privacy. Offer her a consolation that she can pop over each Sunday or whatever at a certain time for a chat with you. But NOT a walk round critique of your home!
Then follow up, if she keeps turning up, like an errant child that won't behave or listen, you then tell her, you'd really rather not have it come to this, but you will change the locks if she doesn't adhere to your simple, understandable request. Then follow through if she doesn't behave as per what you both agree or what you state you expect.
Just simply follow kind, compassionate but firm decisions on restricting her unacceptable behaviour. Tell her she needs to respect boundaries that you are ASKING of her, politely. Hopefully she'll comply and come round, but if not, just follow through with changing the locks. She'll get the message.
But it's best to do it like this as there's no come back on you if you've gently stated what you need and expect to happen. If you do something shocking or blow up, then THAT will blow up in your face as you can be painted as the villain in all this. And she can claim victimhood, which very much suits some women and I'm sure she'll milk it for all it's worth too.