The game is rather aimed at using a controller. Aside from not being able to aim the mortar with the keyboard, going to the "next" page in the Arcade map browser requires using the mouse scroll wheel.

If the cursor loopback is that important (pressing down arrow on the bottom row will move the cursor to the top row again, instead of forcing a next page to load), please consider using some (Page Up / Page Down) keys to move between pages on a keyboard setup. It's quite distracting to browse maps with the keyboard (typing in map names / creator names), then having to switch to the mouse only to scroll the wheel a bit to get a next page.