I think it probably depends on what time of year it is. I seem to remember earlier posts indicating that Mondays were pretty crowded at MK-lots of people travelling on the weekend making the MK their first park. However, we went to MK on a Monday the third week in October and it was fabulous--we almost(No WAY!!!) got sick of riding Splash Mtn over and over at 10:30-11:00 a.m. This was after Space MTn 6 or 7 times with standby lines of fifteen minutes- you could just keep getting fastpasses, though, and walk right back onto the FP line after riding.
If you're worried about it, or it's a busy time of year, be sure to go early, prioritize what you want to do, use FP, and leave for another park if the crowd picks up.