"What is your opinion? Are clothes really useful during summer?"
Translation:Kion vi opinias? Ĉu vestaĵoj vere utilas dum somero?
There are 4 hits for kio estas (la|via) opinio in Tekstaro, but opinii as a verb is far, far more common (>1000 hits for opini\VF). I'm not sure if they'd consider such minor usage acceptable. You can always try submitting it and seeing what happens...
Manĝu terpomojn kaj feliĉiĝu!
I think Carbrule's standard answer applies. It's not accepted because nobody has added it yet. Remember that the Esperanto sentences are primary. Someone must have come up with the sentence "kion vi opinias" and came up with an idiomatic translation in English. Then the system showed you the sentences in reverse order and you (Don) reasonably used a noun in your translation.