You can use Search on sweetfx to see this is raised from time to time. I don't immediately see a ZOS post on here about it but there's a link in one of the threads on here:

http://forums.elderscrollsonline.com/en/discussion/89444/sweetfx-the-enhancement-that-your-eso-game-is-likely-missing/p1

to this thread on reddit where a ZOS employee "Cedric" clearly says it's use is okay:

http://www.reddit.com/r/elderscrollsonline/comments/1zsxmk/my_sweetfx_preset/

Like he said, yes. I use GemFX however, prefere it. See the following posts also

http://forums.elderscrollsonline.com/en-GB/discussion/159349/how-to-bypass-radeonpro-and-make-sweetfx-work-with-esolauncher

http://forums.elderscrollsonline.com/en-GB/discussion/150639/radeon-pro-sweetfx-eso-launcher-graphics-enhancer-info