I believe that the Ceramic tag is not useful. It currently has no usage guidance.
It is rarely used, which may indicate that it's of limited utility. Of the 7 current questions with this tag:
2 relate to ceramic (or PEO) coatings on rim brake rims. First, this arguably falls under questions related to rims or to brakes. Second, this is not a common feature even in high-performance alloy rims. It is not likely to become so, because performance bikes are evolving to use disc brakes.
4 relate to ceramic bearings. These should fall under the tag for bearings anyway.
The last one refers to a ceramic coating on a cassette. Some of these cassettes exist, but they aren't offered by mainstream manufacturers. Ceramic coating is actually relatively rough at a microscopic level. I suspect they would increase drivetrain friction. Mainstream manufacturers may offer nickel or other coatings on their higher-level cassettes to decrease friction.
One counterargument is that there's a clear, if small, market for ceramic bearings on high-end performance bikes. Removing the tag might make it harder for users to find questions related to ceramic bearings, or ceramic other stuff.