Sorry if it's been answered before, but I was just curious: if a ship is already stressed when they attack, does ruthlessness still trigger?

Yep..

It comes into effect if you preform an attack that hits.
It has nothing to do with stress.

Thanks for the reply. A friend and I played a match earlier and I just wanted to make sure that we had played correctly, which we did.

Yep. Cards will explicited state "if are not stressed" if you cant do ot while stressed.