A long time ago, it used to be. It doesn't seem to be now, and I don't have any idea why the distros changed. (Maybe it's due to the mv semantics? But I though people considered the idea of creating a file in /tmp and then renaming it to be bad instead.)
Copying or moving through tmpfs loses silently some file metadata, e.g. extended attributes or high-resolution filestamps.
Copying through /tmp is a frequently used method for transferring files between users on a multi-user computer.
It is said that the Linux kernel will include soon an improved version of tmpfs, which will allow user extended attributes, within certain limitations.
When this will happen, one of the most annoying misfeatures of tmpfs that has persisted for much too many years will finally be gone away.
In fact, I didn't notice the change until now.