The first slogan of Pokemon was "Gotta Catch 'Em All!", so is Ash's last name Ketchum based on it?
Yup,you said it.
Source:My father had read the history of how Pokemon began and he told that "ketchum" was realted to "gotta catch'em all".Its also mentioned in bulbapedia
Yes, it is actually.
He shares his Japanese name—which can be taken to mean "wisdom" or "reason" (智)—with the creator of the Pokémon franchise, Satoshi Tajiri. His English surname is a pun of the original English motto, "Gotta catch 'em all!."
http://bulbapedia.bulbagarden.net/wiki/Ash_Ketchum