If you're talking about football, then DC is a top football town. As a former season ticket holder, I'm shocked that, under the Snyder Regime, there are still as many fans as there are. Since the Redskins have been here 85 years or whatever, you have generations of fans in the area.
As for basketball, the Wizards have a very strong fan base. The Caps have a smaller but even stronger fan base.
As for baseball, a team that's been here for only 14 seasons is naturally going to have a smaller fan base than the Cubs, Yankees, Red Sox, Tigers, whatever teams that have been playing for over 110 years.
By the way, the "no one is from Washington" mantra is mostly BS. I'm from here, and while a lot of my friends and neighbors moved here as adults, most stay for decades. That's the draw of the area - lots of money, strong to decent economy, lots of educated people. Many of them are huge Redskins, Nats, Caps, etc fans. I was a huge Orioles fan prior to 2005, and now I don't even follow them. People do adapt, and if you live somewhere long enough, chances are you'll support the local teams. If you don't, your kids will.