Are Marketo forms responsive? Will they auto adjust their layout to the size of the screen depending on if they are mobile or desktop?
The form layout can work responsively on mobile and desktop depending on how you have it setup. If your form in whole is never wider than your intended screen size (like a mobile at 480pixels), then the form will not differ on mobile devices from its desktop view.
If your form is too wide, you can alter the layout using custom CSS on the form code to make the form fit within the mobile browser screen size bounds.