Ryan Howard is tied for second in the majors with 91 RBI. He has also come to bat with the second-most runners on base of any hitter. Adrian Gonzalez is first in both categories. Does that mean he's a good "RBI guy," or a merely a good hitter driving in a lot of runs because he has a lot of runners on base?
Cleanup hitters are expected to drive in runs. But opportunity is important. If you're hitting behind Jacoby Ellsbury (.369 on-base percentage) and Dustin Pedroia (.402 OBP), you're going to get more chances than if you're hitting after Ichiro Suzuki (.310 OBP) and Brendan Ryan (.322 OBP).
One way to measure the effectiveness of a cleanup hitters is to simply measure the percentage of runners driven. I looked at the 17 hitters who have batted at least 250 times in the cleanup spot this season. Here are the percentages of runners on base that they knocked in (totals are for all plate appearances, not just while batting cleanup), taken from Baseball Prospectus:
Ryan Howard: 18.1 percent (31st among all hitters with at least 250 plate appearances)
Troy Tulowitzki: 17.3 percent (44th)
Matt Kemp: 17.5 percent (38th)
Prince Fielder: 18.4 percent (23rd)
Miguel Cabrera: 14.9 percent (97th)
Carlos Lee: 16.1 percent (71st)
Vladimir Guerrero: 11.1 percent (200th)
Kevin Youkilis: 17.4 percent (40th)
Adam Lind: 14.5 percent (110th)
Paul Konerko: 16.9 percent (51st)
Alex Rodriguez: 17.4 percent (41st)
Carlos Santana: 13.0 percent (157th)
Josh Willingham: 16.1 percent (70th)
Adrian Beltre: 19.2 percent (12th)
Matt Holliday: 20.1 percent (5th)
Aramis Ramirez: 16.6 percent (57th)
Neil Walker: 19.1 percent (13th)
For the most part the spreads aren't that large (although Guerrero hasn't certainly failed to drive in runs -- and sure enough, while he's hitting .275 overall, he's hitting .228 with runners in scoring position).
But you may have noticed another problem here: By just tracking runners on base, it doesn't account for where those runners were on base. Certainly a runner on second is a better RBI opportunity than a runner on first.
There's another major flaw with using this method of evaluation -- it doesn't account for walks. Take Cabrera, for example. According to this statistic, he's only the 97th-best "RBI guy" in the majors. Here's the thing though: He's hitting .373 with runners in scoring position and .348 with men on base. But pitchers will pitch around him whenever possible. With runners in scoring position, Cabrera has had just 102 at-bats, but 31 walks (giving him a .507 on-base percentage), meaning he draws a walk more than 20 percent of the time when a runner is already in scoring position. You think pitchers fear Cabrera?
Compare that to Howard, who has 132 at-bats with runners in scoring position and just 21 walks. (Howard is actually hitting .311 with runners in scoring position). Howard may have 20 more RBI than Cabrera, but it's all a matter of opportunity and how often pitchers challenge you. Or compare to Beltre, who is hitting only .254 with a .296 OBP with runners on base. He also had more RBI than Howard, but rarely draws a walk, meaning overall he's not that productive with runners on, even though he has 76 RBI.
In the end, this is just further evidence of why you shouldn't place much emphasis on a hitter's RBI total. It's simply a tally of something that happened, not a good tool to analyze a hitter with.