Featured images are cropped on index and post page

This is likely a tweak the theme author made to make the images in the theme responsive or adaptive. You may want to look at adding “min-height” to your CSS and this should address your problem.

https://css-tricks.com/almanac/properties/m/min-height/

Also another thing to note if you using “100%” anything the actual dimension is governed by the parent element (whether is width or height).