Like @Kfawcett said, it’s possible of course, and again it might be more effective.
That said, from what I can gather, most bubblers typically do both more or less at the same time.
Bubble development, in my opinion, blurs the line between traditional “front-end/back-end” development in the sense that so much time is saved on both ends that breaking it up and focusing on one or the other as separate projects isn’t necessarily time efficient by any means.
Also, by “designing” the app visually, you are essentially building many of the workflows you’ll need already. It’s just a simple matter of clicking the start workflow button.
If you’ve got a form with a submit button, just create new things for the data and boom you’re done.
What I like to do myself though is spend time defining all my breakpoints and styles first, that way I don’t get caught up when I’m “painting” my app on things like trying to figure out page widths, typography, colors, button corner roundness etc.
And as @john3 says, there is this cascading rippling effect that we run into - as you build the function of the app, other things are going to pop up that could very likely break your design