Clearly, luck is a factor during the regular season, as proven by my "Same Roster Challenge" theme, where we all had the exact same players and ballpark. The range of wins by team went from 66 (low) to 97 (high).... with everyone playing the EXACT SAME ROSTER. The thread summarizing this theme is
here.
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Obviously, a short series (especially the stupid "best-of'5" first round series) means even more luck. I will admit that my personal experience is biased. I used to keep track of every single team I played, the theme, their regular season record and playoff results. I even submitted a post in the forums many years ago, showing all the failed playoff performances by my 100+ wins teams - something stupid, like 2 out of 25 won the World Series (I wish that thread still existed). And also showing how much better my sub 90-win teams did in the playoffs.
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I will concede that the correlation probably isn't negative, especially in open (or progressive) leagues, where the manager skill and/or roster talent differential is even more pronounced. But in theme leagues (which is pretty much all I play in), especially competitive theme leagues where there isn't that much of a difference in managerial skill or roster talent, I bet the correlation is very small. And I still believe the correlation is negative for my teams.