MLB attendance is relatively flat (up .5%) through the first 2 months of the season. Riding the success of last season, Kansas City and Seattle are leading the league in increase average attendance. The Royals made a world series appearance last year while the Mariners gave Seattle residents something to cheer for through September as they took their playoff push to the last game of the season.
On the other end of the spectrum is the Tampa Bay Rays. As of today (June 1st), they are one game over .500 and are tied with the Yankees on leading their Division. Although some locals point to the Tampa Bay Lightning Stanley Cup playoff run, I believe the issue goes deeper than that. The owner is vying for a new Stadium and showing poor attendance is a strategy used to gather support and justification on the request of public funds or getting out of their lease agreement.
Here is a list of the average attendance by team through May over the last 2 years.