Not sure. I guess with a “traditional” bank, cancelling and then re applying for your overdraft every month would probably require a credit search each time which is regarded as a negative on your credit file I think.
Not sure if “switching it off/on” with Monzo has the same effect but probably best to keep it open I think.
If you’re worried that you’ll find it too tempting though then maybe leave it off and only put it on if you absolutely need it, and try and build up a contingency savings pot which you can dip into if needed (I am aware this is easier said than done - believe me I’m in my 40’s and although my finances are ok now, it hasn’t always been this way! If you’ve avoided using your overdraft until now, I’d advise trying to keep it that way if at all possible!