DIY Forms should recognize when they are being viewed on a mobile device. The forms will stack the fields vertically to make it more user friendly on a touch screen or just a smaller screen in general. You may find that the page does not resize properly depending on the last time the page was published.
Responsive DIY Forms was added in the 7.12.0 Release on 9/14/2014. If you haven't already, you will need to Go Live with the form again to publish this enhancement.
Click Management on the menu bar
Click DIY Forms
Click Edit next to the form you are working with
Click Go Live in the upper right corner
If you are using an embeddable layout, you will also need to re-embed the form on the page your organization is hosting.