I don't completely agree that such a healing skill is necessarily bad design, but it could be.
You are indeed encouraging the healer to spam their healing skill (if only used once or twice, it's an inefficient skill since its power is coming from healing repeatedly), and you may be taking away some of the player's strategic decisions by giving them the obvious answer of "who have you healed the most times so far?".
On the other hand, if the player has ways to influence who gets hit by enemy attacks, the enemies deal more damage over the course of several turns than you can reliably heal during those turns, and both this skill and the "tank" characters you'd ideally want to be using it on are both properly balanced so that they're useful without crowding out other character strategies - then I would say this skill might be well-designed, as it creates interesting combo pieces that give your player a way to build up her healing powers before actually needing to heal an ally.
More often in an RPG, I find that limiting healing is the better way to go (too often, healing in games forces battles to be tuned around the busywork of keeping your HP high rather than the joy of finding a way to win), so I might do something like having healing be reduced each time you use it on any character. But this isn't an absolute rule - like I said, there are ways that your skill idea can fit beautifully. You just need to make sure that all of the other mechanics in the game work well with it.
As far as the "how do I implement it", the best way would be to create a new public property for battlers called @times_healed or similar (how you do this is different in each version of RPG Maker, but it's a very simple scripting change in all of them), and reference it in the healing formula (usually b.times_healed) assuming that you're using RPG Maker VX or higher (if using XP or earlier, changing the formula also requires scripting). Finally, at the start of each battle, set all battlers' times_healed property to 0 (another very simple scripting change).