Translation:The mountain is pretty.
'pretty' doesn't sound like the best word to describe a mountain in English, it sounds awkward to me... I mean, the word 'pretty' is better for rather small things, like animals, flowers, sometimes people.
Does it sound similarily weird in Korean, or is it more natural to say that something like river or mountain is 'pretty'?
예쁜 (pretty) is a much more versatile word in korean. While it's most directly translated into "pretty", it can also be used in place of the word beautiful. 아름다운, the direct translation for beautiful, carries significantly stronger connations than 예쁜. If you just want to comment on how something looks nice, you'd say "예쁘다."
Edit: Also, 아름다운 sounds significantly more formal than 예쁜, so in casual speech you'd be more likely to use the latter. (Ironically the example sentence given is conjugated in the highest level of formality.)