Do DC and Marvel hold a US trademark on the term Super Hero? Answer: Yes. It was awarded to them in 1981, and was last renewed in 2002. See
Comic Book Urban Legends Revealed.
Remember that, unlike Copyrights, Trademarks expire relatively quickly (8 or 10 years, I believe) unless renewed, and someone else can apply for a trademark that used to be registered to someone else if it is not currently in effect (which is what happened to Captain Marvel).
The only thing that can't be trademarked are generic terms, or trade names that have become generic instead of specific. One could argue that Super Hero was such a term, but apparently nobody fought it at the time of the original trademark application (filed in July 1979, published in June 1981, and approved in November 1981).