Why is Yoshi named Yoshi

"Yoshi" is Japanese roughly means "YES!" to a certain degree.

This "YES!" resembles Yoshi's upbeat and active nature.

The game developers had created a dino/dragon character with a love for adventure apparently, so they decided to give him a proper name that suits him. Basically, "yes" with a punctuation mark at the end.

Keep in mind though that "Yoshi" does not directly mean "yes" in Japanese. If somebody asks you a yes or no question in Japan, and you reply back to them "yoshi", they will know what you mean, but it's not considered proper grammar.