Not necessarily the wrong time, as after five years, TPTB felt it was time to bring in a new medical student as part of the main cast, and it's often at this point in a show's run that a Cousin Oliver character is brought in.
The main problem is that she was shoved in our faces 24/7--"Here she is, now LIKE HER, DAMNIT!"--at the expense of the other characters--a BAD idea, especially after such an exciting season finale that had me very eager to see what would happen in the fall, only for all the cliffhanger plots to be resolved with a few throwaway lines. Then to cap it off, while she wasn't an especially bad character, she wasn't a good one either.
Ironically, she finally started to catch on in Season 6 (which actually makes perfect sense, as after a year's worth of rotations, she'd be far more experienced and less fumbling and bumbling than when she started), but by then it was too late and they nixed her.
Seriously. Carter, you are A doctor, not HER doctor. People prying into people's private business, especially their medical care, is a HUGE berserk button for me.
Then a year later, when Mark asks him what pain medication he's on--in front of two other people who also had no right to know--he snaps (truthfully, albeit), "That's between me and my doctor."
Funny how he forgot that when he was badgering Lucy.