Where is there good food, any nationality, any type, any expense, and within walking distance from the DC baseball Park?
Outside of the stadium you mean?
Inside, there's a Bens Chili Bowl stand, which is a DC specialty. Get a chili half-smoke all the way, and ask for an extra burned sausage.
Good upscale Italian at Osteria Morini.
Good cheap Korean at Cafe Kimchi.
Good spicy chicken at Nado Peri Peri.
Right outside the gates is the "Fairground" (the parking lot with a bunch of boxcars) and there are usually a collection of food trucks there. It can be very crowded.
Within a few blocks Bluejacket, Justin's Cafe and Park Tavern are probably the best options. I would go with Bluejacket. It is a new brew pub. Their beer is pretty solid, their food tries to be imaginative and doesn't completely miss. I guess you could call it "new american." I wouldn't go out of my way to eat there, but it is not bad before or after a game.
About 3/4 of a mile from the park is 8th St/Barracks Row. If you feel like making the walk, it has much better choices. Cava, Matchbox, Belga, Chesapeake Room and Rose's Luxury, etc ... depending on what you are looking for.