Mrs F's bike has 30mm deep rims ( as measured externally ), and 42mm valves are useable, but only just.
It definitely looks quite 'stubby'.
Depending on your model of pump head, it might struggle to get enough of a grip.
It's fine with all my pumps.
I've just purchased a few longer ( 60mm, but I'd have been happy with 50mm ) tubes to use on these wheels.
So I'd say that 42 is OK at a pinch if it's all you have to hand, but I'd not buy them that size for that wheel.
Over long valves are perfectly fine on shallow section rims, they just look a bit weird.
I've run 80mm tubes on occasion on my normal-section rims.