I tell you I was mad as hell when Mussina signed with the Yankees... I was also 9 years old.
Now c'mon, I don't know how any O's fan could honestly still hold a grudge, for leaving... but we can be a little peeved that it was to the Yankees, just as the O's were being relegated from rival to punching bag for the Evil Empire.
That said, no his number shouldn't be retired. Retiring a number is mostly about the quality of the player, but it also has a lot to do with the connection to the city, Mussina doesn't have that. Part of it is that is that unfortunately he didn't play on any of the championship teams, but another part of it is that he left in his prime to play for the Yankees.