It depends on the party's level.
When the party is at low levels, this is a huge amount of healing that rivals a Life Cleric's Cure spells. By 5th level, the amount of healing is still good, but much less impactful, and by 10th level, a 15 to 20 HP heal is pretty much removing one normal hit from an on-level enemy.
Is it overpowered? It may be, in that it could easily overshadow a character who has chosen healing as their specific role in the party, at first. But on the other hand, a devoted cleric is going to rapidly outpace the amount of healing the Healer feat can produce.
I suspect the Healer feat was added as a way to let a party get along without a cleric/druid/bard in the healer rolerole†, especially in the early levels where characters are much more fragile and have few resources. Later on, it's more likely something you'd use in an emergency if a character drops, and otherwise just use it right before a short rest to help save the party a few hit dice.
That said, it's worth noting that to have the feat at 1st level where it's most valuable, you have to be playing a variant human, and grabbing it at 4th level means you're already moving into the realm where it's becoming less relevant.
Healer is definitely better than potions, but potions are kind of the worst way to get healing in the entire game, so I wouldn't use them as a basis of comparison. At any given level, potions that restore a meaningful amount of HP are enormously expensive, and weaker ones that you've out-leveled aren't worth the action cost of drinking them. (As my playerfriend once put it, "Oh good, I"I can spend an entire action to get back halfhalf as many HP as one hit from the bad guys? I'd rather just attack and try to end the fight faster.") In my games, potions of healing are quickly relegated to the aforementioned emergency situations or used in bulk outside of combat, as in "The fight's over but I have 2 HP left, I'm just going to chug like four potions of healing now."
† In 4th Edition's MMORPG-inspired design, it was functionally a requirement to have a dedicated healer in the party. The classes were built so that healers could generally do "healing and..." every round, but part of 5e's push back towards the 3rd and 2nd Edition feel was to make party composition much less of a barrier to entry -- you shouldn't have to have a specific healer class around in order to have a usable party.