Well, I don't think there's anything wrong with being attractive. It's a genetic predilection, we're naturally drawn to things we consider attractive and beautiful; even something that is physically repulsive can be emotionally attractive, and we may be drawn to it for other reasons. The key to balance, I think, it to temper the physical attraction - which can indeed lead to fetishisation and obsession - with attraction of other kinds, such as appreciation of the character's personality, their narrative role, etc.
I wouldn't call K. Rool or Ridley physically attractive in the sense of being winsome, but they look "cool" and people are drawn to their robust designs and villainous presence. They're also popular because they have a dynamic aura that captivates the player and compels them to remember that sense of awe and admiration, and because they have a certain narrative role as the main antagonist of their respective games, hence the player is familiar with them and develops a sense of attraction towards them from a nostalgic / respectful perspective.