This website actually works really well on mobile and nothing is lost when browsing on a phone. I don't know what an app could do that the website doesn't do better.
As Tasses mentioned above the website does work quite well on mobile and I've seen this issue talked about for quite a while now. I do feel that it would be handy for a ManaCube app but I don't think that's what ManaCube is focusing on right now, possibly in the future.
I think It's a great idea, but what's the point in making an app that will likely not be used when they have a full-fledged website? They don't need to spend the extra funds when there is already a website that's easily accessible.