whether or not the Starfox games with Krystal were bad or not is a matter of opinion. Starfox Adventures had some good elements that most people overlook (great graphics, awesome musical score, and the arwing levels were actually very fun) it just wasn't complex gameplay wise so people think its a bad game all around. Assault was pretty bad because they tried to hard to focus on on-foot combat rather than flight levels, the whole reason Starfox as a series has been good in the first place.
I dunno why people also think Command was a bad game, it was actually pretty fun and critically scored pretty well, actually.
its not the fact that Krystal was in the last 3 SF games that made them "bad." what it really was was Nintendo outsourcing the creation of their IP to 3rd party game developers, who didn't do as good of a job as Starfox 64, which Nintendo made themselves.