Tolkien didn't think of his orcs as fallen angels, at least if fallen angel is considered synonymous with demon as it is in Christianity.
JRR Tolkien,
The Annals of Aman (1958) in JRR Tolkien and Christopher Tolkien,
Morgoth’s Ring (1993):
Orcs we may name them; for in days of old they were strong and fell as demons. Yet they were not of demon kind, but children of earth corrupted by Morgoth, and they could be slain or destroyed by the valiant with weapons of war. (pg 109)
In Letter #71 (1944) he seems to view them as separate categories (emphasis mine): "a motley alliance of
orcs, beasts,
demons, plain naturally honest men, and angels." A similar separation is employed in Letter #131 (1951) (emphasis mine): "elves, dwarves, the Kings of Men, heroic 'Homeric' horsemen,
orcs and demons, the terrors of the Ring-servants and Necromancy, and the vast horror of the Dark Throne." Letter #141 (1954): "Orcs (the word is as far as I am concerned actually derived from Old English
orc 'demon', but only because of its phonetic suitability)." "Phonetic" means the sound of speech, not its meaning.
However Tolkien did regard other entities in his fiction as demonic or even Satanic.
The Annals of Aman: "in Utumno he [Melkor] wrought the race of demons whom the Elves after named the Balrogs." (pg 70)
Quenta Silmarillion (1951-1952), in
Morgoth’s Ring:
Melkor built his strength, and gathered his demons about him. These were the first made of his creatures: their hearts were of fire, but they were cloaked in darkness, and terror went before them; they had whips of flame. Balrogs they were named by the Noldor in later days. (pg 159)
In Letter #153 (1954) Tolkien refers to Morgoth as "Diabolus". Letter #156 (1954): "the absolute Satanic rebellion and evil of Morgoth and his satellite Sauron". In the same letter, Sauron's deception of the Númenoreans is a "Satanic lie".
It can be concluded that balrogs are the closest analogues in Tolkien's fiction to demons, while Morgoth and Sauron are Satanic. Orcs are something else.