I assume you're talking about whether the person running the bot script is mobile or not. You could probably tell if someone is mobile by the dimensions of their screen.
//echo -a $window(-1).w $window(-1).h
Whether or not someone is mobile, I assume the real reason you're checking is that mobile has smaller screen, so checking for this would identify mobiles with small screens, but probably not identify those with large enough dimensions that they don't need to have alternative content spoonfed to them. It is possible but not likely that someone will be running mirc on a desktop with dimensions reduced enough to fool this into thinking it's mobile.
