Agreed, the default form state really leaves something to be desired...
I did a lot of styling to try to make the forms look better using HTML in the actual template itself. It stands to reason those types of tweaks would probably work if you inserted CSS into the HTML drag and drop.
Using CSS in the template to override default Marketo form styles, I've been able to change field length, change default form width, change padding/margin around input fields, and even make the length of the page expand so that if form errors appear when someone doesn't fill out the entire form they don't run off the page. (I basically had to start from scratch and completely restructure the entire Marketo template in order to do that, though.)