I don't know how much of a factor it is in gauging the depth of baseball interest in a particular city, but the Portland Sea Dogs are 20th in MILB attendance, which is good for an AA club, but doesn't necessarily point to a deep interest in baseball in Portland.
Among teams in larger markets, the Nashville Sounds are 4th in attendance (29th largest TV market), the Indianapolis Indians are 5th (27th largest TV market), the Charlotte Knights are 7th (22nd largest TV market) and the Las Vegas Aviators are 9th (40th largest TV market).