shakey lawsuit is shakey
The internet wayback machine shows M&M using 'Mighty Mouse' in 2004, while Apple didn't start until 2005, but...
Mighty Mouse trademark was owned and abandoned in 2004 by one William T Wilkinson of New Jersey. He also owned the TMs Mighty Mulch and Comfort Mouse. Trademark squatter? I don't know.
M&M's TM application wasn't filed until Dec 18, 2007. CBS applied for the TM in respect to computer mice on Jul 9, 2007.
My guess is that M&M didn't bother with legal action until their use of the mark was threatened. Now they're fighting it. I think that they would have been wiser to fight it in 2005 - they may have lost their chance now.