I think the MCAT score is more of a factor in terms of med school success, not really how many times you took it. Ex: 1st time, 28 and 2nd time 33 is better than somebody who took it once and got 30.
It also matters what you were doing while studying for MCAT. Some pre-meds are taking a full caseload in college & lets say get a 34. That person will likely do better than another person who took the semester off and pulled a 34.
Students who retake the MCAT are as a whole weaker students than one-time takers, so there is an ASSOCIATION btwn retaking MCAT and doing worse in school, but I don't think retaking the test CAUSES somebody to do poorly in med school. Thus, retaking the test is a confounding factor.