Des Moines may be the largest city in Iowa, but even so, the Des Moines metro area is just over half a million. With a very dedicated fan base, it could support a pro sports team (Green Bay metro area is under 300,000), but typically even metro areas with twice as many people (Memphis is a good example at about 1.2 million) have trouble supporting professional teams.

If you really want to put a pro sports team in Iowa, the best place would either be on the east side of Iowa City, or maybe just west of Davenport. With either of those locations, you could easily pull in both the Cedar Rapids/Iowa City population (a little over 400,000) and the Quad Cities population (a little under 400,000). Of course any pro sports team in this area would be competing with the Iowa Hawkeyes, which is a problem that the Memphis Grizzlies have with the University of Memphis Tigers.

The only major professional sports team to ever exist in Iowa was the Waterloo Hawks (unrelated to the current Atlanta Hawks) which played in the NBA for only the 1949-1950 season.

(All population figures from Wikipedia, trust at your own risk.)

This is a really nice capital city in a prosperous part of the country. Iowans are into football, but Des Moines would be marginal at best.