Not necessarily. The only way an astronaut could've fixed Galileo's antenna is if the antenna was deployed before the IUS fired or if the mission were manned. Could you imagine how much it would've cost to mount a manned mission to Jupiter? Instead of spending about $3 billion, the mission would've cost more like $300 billion. In that case, there wouldn't have been a mission in the first place. Galileo's antenna would've needing fixing had the vehicle been properly checked out before launch. The antenna was damaged during shipment.

As for Mars Observer, that was a pretty subtle error. It spacecraft was about 40 miles off course after flying some 400 million miles. Unfortunately, that error was in the wrong direction. Had the spacecraft been 40 miles too high instead of 40 miles too low, the mission would've probably been successful. It's true that a human crew might've noticed that they were coming in too low. However, they might not have realized it until it was too late to maneuver.

I'm all in favor of crewed missions to other worlds but realize they're a lot harder than it seems. If we're having all this trouble with a space station in LEO, imagine how much more difficult it'll be to send people to Mars. Odds are a Mars vehicle will be many times more complicated than the ISS because of the self-sufficient life support system, navigation and propulsion systems, lander, etc. Like I said before, the more stuff you have.... Maybe we need to send the crew to compete in "Junkyard Wars" to hone their repair skills.