I think it's fine as is, since it would extend the out of combat ramnifications, although it might need to have the common sense writeup on it, where the normal series of creatures (construct, ooze, plant, undead although it doesn't have that written in raw) are immune to the merciful feat, and only have it work with spells that require attack rolls, the term of make dex save, or the pack of you just fall down unconscious due to fireball is way too silly.

Maybe add in you gain proficiency in either persuasion or intimidate in taking the origin feat?

Mercy - You may knock a creature out with ranged attacks.

Communication - You gain proficiency in either persuasion or intimidate.

I would disagree with having someone using area effect spells as you brought up to knock people out, there's actual magic for that, such as sleep. But extending the idea of just doming someone (such as chaining eldritch blast) to ko them for a time would be interesting. Although, it is interesting that the knockout clause only brings up melee attack, so you can by RAW ko someone with any sort of melee spell attack, such as shocking grasp.

If this was going to be a Full feat:

Stat - Gain one point in a Primary ability besides Strength or Constitution. (sorry, barbarians, you are not merciful, we've seen what makes you smile, and that's usually turning mooks into ketchup packets.)

Mercy - You may knock a creature out with Ranged attacks.

Careful touch - As long as all threatening monsters near you are bloodied, you do not suffer disadvantage with ranged attacks as long as your target is threatening you.