What does the screen resolution have to do with it? Genuinely not sure what the relationship is. I could imagine sharp corners appearing more hostile when the pixels are smaller?
Higher ppi help for rendering curved lines cleanly without jagged anti-aliasing.
It's a known fact that typefaces that have curved, humane shapes are more legible. That is why it is bad practice to use something like futura for long text.
I am assuming rounded corners in boxes follow that logic.