It's actually two touches; remember that touching the North indicator is the quickest way of accessing the Chart menu.
Obviously a very good reminder

We talked in the office about enabling mountain terrain in planning mode, but without your current altitude, what altitude should it use as its threshold?
Definitely not easy to answer.
At first, I would have said the lowest altitude of a leg (meaning than planning a route is a prerequisite) but it certainly implies that the Mountain view layer is "sectored" which is challenging (not to say more) from both UI and programming points of view. Not speaking about the mental efforts you need to understand the representation (relative vs absolute)!
Perhaps you could use the default level of every plane profile? I's not perfect but at least it's consistent with the virtual radar behaviour. The drawback is that it does not address the climbing and descending phases.
Well ... perhaps keeping the mountain view an inflight tool only is a good thing, since not showing an info is better than showing a wrong / incomplete or unclear one

M