The website works totally fine on my phone :) actually typing this from it. You just have to scroll a lot to get to the boxes that are on the right and left of your screen when on a computer but that's not a big problem at all really.
I don't think an application which is just the cubecraft forums for mobile users with a small screen is needed.
However, when I started thinking about an app for cube, I started liking it a lot. I figured the app would contain a "player of the day or week", which could be the player that won the most games or got the most kills in that week.
Also thought a video spotlight section, spotlighting videos made on cube and a fan art section.
Of course itd make sense to add the forums as well.
Now this is not a suggestion (What I wrote) but just an idea :) there'd have to be a whole staffteam just for the app, choosing the videos, arts and players to be displayed.