MemberMouse provides the functional components of the order form, but MemberMouse Core Pages, such as the Checkout Core Page, are setup as standard WordPress pages with special function in MemberMouse. Thus, ultimately the styling and mobile responsiveness of your order form will depend on the WordPress theme you're using and any custom styling you may have added. 

MemberMouse provides very little styling of its own, and this is mostly functional.  If desired, the styling MemberMouse does use can be disabled via the instructions in our Use Built in CSS on Core Pages article.