to "reveal something" is to make it visible or make it known. For something to "come as a revelation", it must have been previously hidden or unknown. To say that "something comes as no revelation" means that it was already well-known. And so the assertion that "the notion that clothes do more than passively shroud and protect a person’s body but become part of their psychology comes as no revelation" says that no one would find the notion novel or particularly deep. Instead, as the author says, it is a "facile observation".

"Comes as" just mean "is" here. Literally, "comes" could be interpreted here as "is received."

A revelation is like a secret or surprise. Sometimes people say, "it comes as no surprise". It means that they weren't shocked or surprised by what was said or what they saw. They expected it.