Mum may mean well, but if she's making critical comments as well as taking jobs on herself, then, imo, she IS being judgmental. I'm not sure why you "can't say anything" just because she is "sensitive." What's the worst that will happen? She'll cry? Scream and yell? Storm out angrily? Sulk and stay away for a couple of weeks? I know you don't want any of this to happen. But if it means she stops doing things without asking, perhaps it would be worth it?
You don't have to give her a whole big speech. Maybe try just stopping her in the act when you catch her? (Never mind that, Mum. I'll take care of it.") Or saying "No thank you," when she shows up with a lawn mower, etc? (If she complains that she hauled it all the way over to your house, you can remind her - gently - that you didn't ask her to.) She may be hurt and angry in the moment, but I bet she'll never do it again.
If you can't bring yourself to be firm with her, then I guess you'll just have to laugh about it in your head ("There she goes again, lol!") and enjoy the extra time it gives you to do other, less mundane things.