The long answer is you SORT OF can, but it might not be worth it, anyway.
You could put all your events in a group, then add an event outside the group to toggle the group active every tick.
But the behaviors will still run as if it were 60fps, which could cause a lot of unwanted results. You could add events that set movement-related behaviors enabled or not enabled every other tick, and if you double all your behavior variables (max speed, acceleration, etc.), it will be almost the same as a game running at 30fps.
Even then, though, you could have some unintended consequences. This method essentially makes the behaviors act as if the "off" tick takes as long as the "on" tick, and if that's not true, you could have problems.
Finally, all this only affects your game logic. Graphics will still redraw at 60fps, unless you're doing event-controlled rendering, like with R0j0hound's Paster plugin.
The only real reason to make a 30fps game is to save computer resources, and trying to get 30fps in Construct 2 probably won't save much processing power. Construct 2 will always try to run at 60fps, so unless you put in a lot of planning, you'll have one tick of next to nothing happening, followed by one tick of the entire event sheet trying to run. With the movement timing problems that are also likely to occur, getting 30fps in Construct 2 is probably more trouble than it's worth.