Is it bad to withdraw a manuscript one day after it has been submitted?
I am a co-editor of an interdisciplinary journal (social sciences/humanities). I would rather an author withdraw and get the piece into shape than for us to either read it and have to desk reject, or to send to reviewers and they flag it. It's mildly embarrassing but like so many things in academia (and life), you will care more than they will care – it is trivial for me to return a paper to an author.
If the editorial management system allows you to withdraw the paper, even better.