Shigeru Miyamoto has spoken many times about the process of perfecting the controls for SMB, which is quite the opposite of "wasn't even trying".

Oh he absolutely cared about the programming and process of game development, don't get me wrong. It's that Nintendo (corporate) needed something for the NA market and he was eager to fill that gap with already used characters (such as Mario). Anything else is corporate propaganda they have been telling as a creation myth to make it sound more fairytale-like than it was. They were going broke. They needed money. North America was ripe for the picking. The risk paid off.