Is a blowtorch suitable for browning/caramelising steamed food?

Browning with a blowtorch is the standard method for finishing meat and fish cooked sous-vide. I see no reason why it couldn't also be used for microwaved or steamed dishes, with the caveat that dishes that have too high of a water content will never brown properly (instead, the surface will pop and boil). I'd advise experimenting a bit, to see which side of the "too much water" each dish is on, and whether simply blotting the surface dries them enough for browning.

UPDATED TO ADD: I'm also going to advise you to get a hardware store blowtorch, not a culinary blowtorch. A standard plumber's blowtorch is cheaper, and has significantly more flame and a larger fuel tank.

A blowtorch can technically be used for these tasks, but it'd be annoying and inefficent to do so.

The basic issue is that a blowtorch is too hot and transfers too much heat. Quick heat transfer is exactly what you want for creme brulee, where you need to caramelize the sugar on top without overcooking the custard underneath. But the foods you mention don't brown from caramelization, but from the Maillard reaction, which takes place at a lower temperature over a longer period of time (technically at higher temperatures too, but you wouldn't notice because you're also turning it to charcoal). If you turn the blowtorch on a microwaved steak, you can easily put a black crust on parts of it, but it'll be difficult and time-consuming to evenly brown it.

It is possible to get a "heat spreader" for a blowtorch, which might give you better results. Still, plan to burn a bunch of food before you get your technique down.