In Japanese culture the secondary characters for their comics and animation tend to be bland as Precia-T is describing here and in some series are even entirely faceless. In the animation industry, huge companies like Disney, Pixar, Dreamworks, etc. generally follow the same formula of having unique main characters and generic, "normal" secondary characters. There are a few exceptions to the rule, but they are simply exceptions.
The same can be said for people that wind up liking secondary characters instead of the main because personal taste will vary, but they are still an exception. You even see this technique in live action, especially in the superhero movies. It's a technique used to draw more attention to main characters by way of emphasizing the traits of the main and deemphasizing the traits of everyone else.
With that said, I do believe you can have a fantastic series that breaks the rules here. One Punch Man totally plays with the tropes thanks to Saitama!