I think you've rounded off both lists quite nicely. It'd be difficult for me to distill the last 30-some years of gaming into just four mascots, but Mario, Sonic, and Pac-Man definitely need to be on that list for changing the landscape of video gaming forever. Somewhere in the list of most forgettable mascots would need to be Alex Kidd. Despite being heavily pushed by Sega in the late '80s, Alex Kidd possessed nowhere near the draw power of Mario and was relegated to obscurity once Sonic debuted on the Mega Drive. I don't think he's a particularly offensive character, mind you, just terribly plain. Later Alex Kidd games seem to suffer from an apparent lack of interest on the part of the developers, as at some point, it feels like Sega legitimately stopped caring.