Spoiler text can't be made visible on a touch screen.
(SE seems to be using the same definition of "mobile" as Facebook does: a little phone device. These are not the only devices that are mobile!).
This is not a good solution. If I have a large-screen touch device (an iPad, an Android tablet (although this might work already and there are only about 10 of them anyway), a Windows 8 touch-only device, a Kindle Fire, a TouchPad, etc) then I don't want to use the "mobile" version of SE - it looks like crap on a large screen.
Please provide a way to unspoiler text on the real StackExchange sites.