I have doubts about Asgore, I'd say he's more Lawful Neutral, Blooky's more True Neutral and Muffet would be Chaotic Neutral (though Mettaton is definitely a solid fit there).

Where would Undyne, Alphys and Toriel go if they were on here, would you say?

There is really only 2 Evil characters, Flowey and Chara so a third would probably be needed, I think the reason people say Mad Dummy is evil is because not only is he a boss, but he attacks you in a fit of rage more than anything. (Plus the fact he tries to outright stab you with a knife does not help)