Did Melkor really mar creation?

[Man of the Old Hope]

I think this passage from a draft of a letter to Peter Hastings (153 in The Letters of J.R.R. Tolkien) gives us some idea of how Tolkien viewed Melkor's "creative" power:

"So in this myth, it is 'feigned' (legitimately whether that is a feature of the real world or not) that He gave special 'sub-creative' powers to certain of His highest created beings: that is guarantee that what they devised and made should be given the reality of Creation. Of course within limits, and of course subject to certain commands or prohibitions. But if they 'fell', as the Diabolus Morgoth did, and started making things 'for himself, to be their Lord', these would then 'be', even if Morgoth broke the supreme ban against making other 'rational' creatures like Elves or Men. They would at least 'be' real physical realities in the physical world, however evil they might prove, even 'mocking' the Children of God. They would be Morgoth's greatest Sins, abuses of his high privilege, and would be creatures begotten of Sin, and naturally bad. (I nearly wrote 'irredeemably bad'; but that would be going too far. Because by accepting or tolerating their making - necessary to their existence - even Orcs would become part of the World, which is God's and ultimately good.)...I have represented Orcs as pre-existing real beings on whom the Dark Lord has exerted the fullness of his power in remodelling and corrupting them, not making them. That God would 'tolerate' that, seems no worse theology than the toleration of the calculated dehumanizing of Men by tyrants that goes on today. There might be other 'makings' all the same which were more like puppets filled (only at a distance) with their maker's mind and will, or ant-like operating under direction of a queen-centre." (emphasis added)

Two themes emerge here, and we can see at least the second in the Ainulindalë. First, the notion that Melkor/Morgoth's creative power is derivative and corrupting; and second, that even this corrupting power is ordered finally toward the purposes of Eru Ilúvatar: "And thou, Melkor, wilt discover all the secret thoughts of thy mind, and wilt perceive that they are but a part of the whole and tributary to its glory."

If there be any beauty whatever in Melkor's works, it is so only because of the thought and providence of Eru Ilúvatar.