I just linked to
a whole investigation into the saying.
“A game’s only late until it ships, but it sucks forever.” --Siobhan Beeman, Computer Games Developers Conference 1996 Proceedings
“A good game is only late until it ships, a bad game is bad forever.” --Jason Schreiber, Gamefan issue for June 1998
“ 'A delayed game is eventually good, a bad game is bad forever.' - Shigeru Miyamoto.” --Usenet post on alt.comics.2000ad from October 4, 2003
Like a lot of quotes found on the Internet, it only really got traction after getting transformed and misattributed and it's this transformed and misattributed version that became "canon" in common consciousness. No, Miyamoto is not the originator of that phrase, and in fact there's no record of him ever saying it.