A user on reddit asked this once and he was not wrong, because
emacs does seem blocky compared to modern designs, which often employs
rounded corners, e.g for buttons:

https://www.webdesignerwall.com/wp-content/uploads/2010/04/button-preview.jpg

It could improve the square apperance if, for example, emacs provided some text
properties to specify rounded corners with custom radius for background colors.