Who gets to decide the level of importance?

Objectively? I don't know or even consider myself as someone who can answer that. Subjectively? Read below.

Are you saying that scientists should only work on projects based on some kind of hierarchy of importance?

I don't know if I would call it importance, but there are problems with higher stakes. This also can vary per person. For the sake of argument, let's say that mankind's survival would be my most pressing goal. Stakes are high, as without some way to perform exodus and colonise other worlds, our whole existence could be wiped-out easily. If this would be my view of things, a phone or microwave harming someone would barely register as far as humanity is concerned.

That's hardly my view and I would rather veer away from anything involving ethics, but to me there is nothing prohibiting existence of some scale of importance. As I have said before, I don't consider myself an authority. My gut feeling is simply "hard problems don't have to carry much significance, but it's worth to try and solve them if possible".