why is "she is pretty but poor" wrong?
Krásná (beautiful) is stronger than hezká or pěkná (pretty).
"pretty" is less beautiful than "beautiful". It would be "Je hezká" or "Je pěkná".
I would suggest accepting also "She is beautiful, yet poor."
Technically that's more "Je krásná, přesto chudá", but sure, I can add it, it works.