Is it illegal to cover a smoke detector?

If you mean to interfere with a smoke detector on an airplane, that would be a federal offense. If you cover up a smoke detector in your own house, that would be not be smart to do but is likely not illegal. For rental properties or office buildings, you would probably have to check local building and safety codes to find out if it is illegal, but covering one there would not be a smart thing to do. Many codes in public buildings require working detectors.