What's so funny about A grasshopper walks into a bar?

There is a cocktail called Grasshopper.

So the joke is,

The bartender asked the grasshopper, did you know that we have a drink named after you?

Grasshopper replied, "Why did you name the drink Bob?"

The name the bartender mentioned was named after "Grasshopper", and Grasshopper's Understanding is named after himself. This grasshopper's name is Bob.