I don’t think it’s a good idea as it could cause you to become easily overdrawn with your main bank. At least doing it manually means you can check your balance before moving money over.
Also worth remembering the pre-paid card is only a temporary product until their full bank account is launched. Features that aren’t useful in the context of a proper bank account probably aren’t worth spending developer time on. Once it’s a full bank Monzo would hope you’d use it as your main account and therefore the principle of ‘topping up’ doesn’t really apply once it’s a bank account.