Sounds to me like he's just saying how much he hates Americans in general, by naming the things that are considering typically and undeniably American. It's like saying those stupid French people with their wine, baguettes and their refusal to speak english.
Why does he hate apple pies? Because it's an American thing. Why does he hate baseball? Because it's an American thing. Why does he hate American children? Because they're American. Or maybe it's a reference to the book 'As American as Mom, Baseball, and Apple Pie' by Lindo J. Holland-Toll. Which is of course a reference to the famous phrase As American As Apple Pie.
Richtofen calling out Maxis's Accent
This one is simple. Richtofen hating Maxis's accent means that they are both from different regions of Germany. My German teacher for instance never ever shuts up about how people in the east of Germany have a much more calm, smoothing and "prettier" accent compared to the south, who speak a lot more faster and fiercer according to her.
Interesting read nonetheless.