The title says it all : Is there a way to change a mask's opacity ? I see that the opacity setting in the layer manager is greyed out when you select a mask so I guess the answer is no but who knows, I could be missing something.

No -- that's mathematically impossible. But you can fill the mask with, say gray 125 to get it work at half strength.

To be clearer, this mask opacity (which to my memory exists in Photoshop) would be nothing more than a multiplier of the existing mask (from 0 to 1). The advantage in having it as a slider is that it would be non-destructive and more interactive, allowing for more fine-tuning.

If such a feature ever gets added, we could even imagine a checkbox to invert the effect, turning the multipler into an added value (still from 0 to 1, clamping at 1). That's something I remember missing from toshop so it could be one more thing Krita does better.

Do you think I should make a feature request for it?

It's usually better to have a broad discussion with a few other people before making any kind of formal feature request.

There are a few active discussions about new features and modifications to existing features here: https://krita-artists.org/
so you could sign up, join in and start your own if you like.

I added a topic there but here's the thing: Colorize masks and filter layers already have a working opacity slider. Only some masks have it greyed out.
I can't imagine any good reason to keep it greyed-out.

Anyway, here's the topic: https://krita-artists.org/t/un-grey-out ... masks/6687