She is an "affordable housing" talking parrot. There is some merit to questioning the multiplier effect produced by an NFL stadium vs, say, baseball, and municipal bonds probably aren't looking very attractive to investors rn...but I don't think that is where she is coming from.
Yeah, the "only 8 games a year" argument is used a lot against NFL stadiums, but this one is largely being funded by Harris & Co., so a bit harder to make the argument that the stadium will cost DC money.
And even the 8 games a year argument is specious. It's more than that alone when you add in preseason, plus concerts, pro wrestling, soccer, college games, etc. And, potentially playoff games.
One thing I didn't realize until recently is that stadiums get used quite a bit for corporate events. I was out in Las Vegas a few weeks ago for a conference that rented out Allegiant for an attendee concert. Granted Vegas is a different beast, but there are a lot of events in DC that could make at least some use of football stadium for activities. M&T Bank stadium even has a wedding venue.