In a word, no. September games, on average, are less important than games in May, June and July. Here’s a table of the relative importance of games by month, from 2008:
Month Game LI Apr 0.47 May 0.63 Jun 0.70 Jul 0.64 Aug 0.46 Sep 0.43
As you can see, games (on average) were less critical in the last two months of the season. Now, there were some very important games in 2008, including two games of playoff intensity between the Twins and White Sox. There were a couple of division races, too.
Still, the criticality of those games was more than offset by the number of teams out of their division races, as well as the number of teams with comfortable leads. No drama for those teams; no critical games. On average, less intensity at the end of the season.
I chose 2008 because that was the year I pulled together my Drama Index for games, which measured the criticality of games based on each division race. You can argue that tighter pennant races will result in different results, and you’d be right. But the monthly differences in 2008 aren’t small, and I doubt they’d be overcome by even the most intense pennant races. Maybe someday I’ll run the results for 1967.
The point is, as Colin Wyers noted on Twitter this morning, that September results in general matter less for your MVP votes than other months. Keep in mind, however, that importance will vary by team.