Mercy is great, and offers much more effectiveness/utility at levels lower than diamond/master than other healers. Yeah, Ana and Baptiste offer more HPS, if your positioning isn't shit and you're not wasting your cooldowns or missing shots. Yeah, discord is better if you're not (again) positioning poorly and getting picked off with the biggest fucking healer hitbox in the game.
Mercy gets a free repositioning button, and while you can technically waste res, there aren't too many situations where ressurecting someone creates a worse outcome than if you had saved it. Where as you can definitely fuck over yourself and your team with a bad bionade, immortality lamp, amp it up, and even fade away. Plus, any one trick worth their salt knows the GA tech, so Mercy is probably the most mobile healer aside from maybe Lucio.
So no, Mercy isn't subpar, and one tricking her isn't bad because chances are, you don't want that individual on another healer.