Responsive features (being able to adapt to other screen sizes) is horrendously broken on just about every level I can think of. The core for the new LyokoBoot revision, FreshBoot, is being designed with a lot of emphasis on mobile browsers to ensure that everything works as intended on all devices.
As for the tag line not showing on mobile, technically the entire header should not be showing up on mobile phones. I won't have access to a computer until tonight, most likely (still 30 miles away from home, I think) but I'm going to add a CSS class to it so it behaves more nicely.
If you want to see my FreshBoot thought process, take a look at my screenshot thread. Alternatively, if you want a native forum browser for your whateverDevice, Lyoko Freak supports the Tapatalk forum browser. Tapatalk 1 should be free on all of the major app stores, and IMO it's much nicer than 2.