You Americans all want to think that Toad, Yoshi, and Birdo are unique individuals, but they actually are not.
The Japanese have a concept that a person or creature can simultaneously be an individual as well as every member of his or her group. That's what you see with these characters. Every Toad that you see has a mushroom head, wears a vest, and has the same personality. Every Yoshi that you see, they're all named Yoshi, they all spout gibberish, and they all have the same personality. Every Birdo that you see is named Birdo, wears a bow, ring, and makeup, and has the same personality.
The Japanese makes no distinction that any Toad, Yoshi, or Birdo is a unique individual from the rest. They are all one and the same.