The series images that appear in the Top Shelf section of the Home Screen on Apple TV are cropped resulting in some of the image being cut-off. I know, I'm nitpicking here, but I can't unsee it. Is this something that can be easily fixed?
No. This is a long-standing issue, and the devs are aware of it.
(Although, I dare say it's far less egregious than the cropping of movie posters in the On Now tabs, which makes them nearly useless.)
Out of curiosity, what’s causing this and what are the limitations to fixing it?
Nevermind, I see it's due to limitations with the tvOS API. However that was back in 2018. Have there been any updates to this since? Fubo shows series/movie images in the top shelf and those appear to feature the correct landscape dimensions. And Infuse is using series/movie images in portrait mode.