Couple of Trademark laws caveats
1: CBS could, but probably did not, trademark the name Mighty Mouse in the case of electrical devices (which is the likeliest category for such a trademark).
Trademarks are granted in specific areas, and you can reserve a trademark in any area you want. If CBS did not trademark it in the proper category, then M&M could have. And CBS scammed Apple in "giving them rights" to a trademark they did not have. That's likely to be the case.
2: If I remember right the US legal environment, there's a specificity in trademark law that's called "due diligence". You are supposed to enforce your trademark against infrigement.
Apple has been selling its "Mighty Mouse" for years now. It's hard to claim by Man & Machine that it was unaware of the violation - Apple is not exactly a low profile provider of computer equipment. If Man & Machine did not sue as soon as they became aware of the trademark violation, they implicitely agreed not to, and lost their rights.