The IMDB calls A League of Their Own "The highest-grossing baseball movie." Not sure if this is true, but I can believe it. I imagine baseball movies is a niche genre, so if you can expand that niche, as A League of Their Own does with its feminist undercurrents, you might gross more at the box office than, say, Major League. I remember enjoying A League of Their Own when it came out, and revisiting it more than 30 years later, it retains its enjoyable nature.
Is it a classic? Maybe a classic baseball movie, but I don't know that I'd go further than that. There are some fun performances ... Madonna was never better in a movie, and I always like Lori Petty, while Geena Davis is iconic and Tom Hanks is ... well, he does get to say "There's no crying in baseball" and that's one of the most memorable quotes in movie history. The recreation of the All-American Girls Professional Baseball League is important, even necessary.
But there is nothing revolutionary about the approach of director Penny Marshall or writers Lowell Ganz and Babaloo Mandel. It's just Yet Another Sports Movie where our heroines rise above the barriers placed in front of them, leading to an inspiring and tear-jerking finish. It matters that the barriers in this case are historic and that the stars are women. But the revolution ends there, which limits the film to something more enjoyable than great. Of course, there's nothing wrong with enjoyable ... there's a reason for its continued popularity.