I'm sure you never wanted to think about Cedeno's 2006 season again, but there it is, along with Castro's current season. Despite a better offensive batting line, Cedeno's wRC+ is worse because the league offense was higher in 2005.
Starlin Castro hasn't been as bad offensively, but he's been far worse defensively. In roughly 250 fewer plate appearances, his baserunner has been equally bad. They both played SS so they had positional adjustments on their side when calculating WAR. Despite that, were so bad that they were more than a win worse than replacement level.
Among qualified Cubs hitters since 2001, Ronny Cedeno's 47 wRC+ is the worst. Castro's playing time is considered to be qualified since we're not even to the halfway point yet. If he never played the rest of the season, he wouldn't be.
Including the 2013 qualified Cubs hitters, Castro ranks as better in wRC+ than only Ronny Cedeno since 2001. Both the in-season ZiPS and Steamer projections project Castro will finish the season below replacement level.
ZIPS projects he'll end the season with -0.4 fWAR, which would put him better than only Cedeno in WAR among qualified hitters since 2001. Steamer has him finishing with -0.2, which will tie him with Alfonso Soriano, better than only Cedeno.
Unless Castro is significantly better than the rest of season projections, he's likely to finish the year as having one of the worst seasons by a Cubs position player since 2001 (among qualfied players). The thing is, if you suck this much, you usually don't get that amount of playing time, but Cedeno was young and so is Castro. Castro has also shown he's much better than this.
I didn't want to stop at 2001 so I looked back to 1991. It didn't change Castro's current ranking.
If you go back to 1981, Ivan DeJesus' 1981 season takes the worst spot in wRC+ (42). Castro is 3rd worst. What amazes me about DeJesus' 1981 is that he had a 10% walk rate and only struckout 13.3% of the time. He also carried a .227 BABIP, which is a big reason why he was so bad.
As I mentioned, Castro has half a season left to play and his numbers should improve. Castro is still going to have one of the worst seasons among qualified Cubs position players when it's all said and done, but he should be able to brag that it's only the 3rd, 4th or 5th worst.
I went back to 1951 and found that Castro is now 4th behind Don Kessinger among qualified Cubs position players.
Since 1901, Castro currently has the 5th worst wRC+ among qualified Cubs.