In combat having a 1d4 healing cantrip as an action wouldn't be game breaking, and as bonus action would be the best cantrip in the game. But the real problem is outside of combat. If you just spam the cantrip to heal everyone then:
- Prayer of Healing becomes a useless spell.
- Goodberries becomes a useless spell
- Every food instantly becomes just junk to be sold.
- Every basic healing potion becomes just junk to be sold.
- Short rests lose a part of their value.
And probably a lot more I can't think of, just the sheer amount of gold you would make by having this cantrip would make it the best in the game by far.
