Similarly to the startup community, where everything's been done, "clones" are OK in this sense. Dissimilarly, you don't have to be the best with an indie game, you just have to have some fans.
Terraria takes inspiration from Minecraft, but it is not a clone of it.
Zynga's "The Ville" is a clone, for reference. [1] Terraria is what happens when a dev sees a neat game mechanic and thinks "Oh wow, I know what I can do with that" and extends it, which is to be embraced. It's how whole genres are launched and how we got some of the greatest games in the industry.
Notch has stated that he originally wanted a game like Dwarf Fortress in tone before he stumbled upon Infiniminer[1]. I think you are right in that it's more correct to say Minecraft is a clone and evolution of Infiniminer, but was heavily inspired by Dwarf Fortress.
