In that case, wouldn't it be more effective to throw Mozilla's weight behind an open source mobile Operating System that actually has a chance of going mainstream ala MeeGo?
It depends on the goal. If the goal is to have an open source mobile operating system, then maybe. If the goal is to have it not matter which exact mobile operating system you use, so new mobile operating systems have an easier time entering the market.... then no.
In case it's not clear, Mozilla is not building a kernel or anything here. They're just building capabilities for web apps to be able to do whatever mobile apps can do now.