Yukio Mishima is one of the acknowledged by all masters of post WWII Japanese literature. I have read all the others novels in The Sea of Fertility Tetrology, several of his plays and short stories as well as other novels. Post WWII Japanese literature is one of my core interests. (There is background data on Mishima in my other posts on him.) He and Kenzaburo Oe, who I do put above him, are my highest regarded Japanese authors, so far of the 100 or so I have read.
The Decay of the Angel probably will only be read by hard core devotees of his work who have read the first three books in The Sea of Fertility Tetrology. It is a book only for those really into his work. It is the most "philosophical" of his books I have yet read. At times it did seem he was cudgeling me with his views on beauty, death, cultural decay and the decadence of society.
I have said before that the dominant theme of the post World War Two Japanese novel is the impact of Japan's defeat in the war. Japan did not just lose a war, the fundamental belief structure of their society was destroyed and nothing has emerged to take its place. This is what is behind Mishima. Mishima is an artist, not a rigorous logician and if pushed his ideas may emerge as incoherent but it is the incoherence of the Sea of Fertility in 6000 year old near atavistic faiths.