We are well aware of the need for responsive design; in fact, we're redesigning our site and Seller Admin with this in mind, and there should already be nothing inherently unresponsive about our current button codes or cart.
Our box.js script, which is normally loaded into your page as part of your standard View Cart code, manages our overlay-style cart display "inside" your page and detects when the buyer is using a mobile device; in that case, your cart buttons would work in "fallback" mode by opening a separate window/tab to display the cart (rather than displaying the overlay inside your page), to take best advantage of the available screen size and support the standard touchscreen pan/zoom/scroll gesture functionality. I did just now spot a couple minor tweaks we could apply to significantly improve the fallback cart's layout responsiveness, so I've sent that request to Development.
If your site is displaying the overlay-style cart inside your page when viewed on mobile devices, please reply with the URL of a page where that is happening and provide any details you can about which device and browser app produces that problem behavior.