-네 is used to express surprise or wonder.
크네: (it’s) huge!
Very useful, thank you!
So it's an extra "exclamation mark" that affects syllable count rather than statement intonation.
"The cloud is very beautiful" should also be correct.
Trying to line up Korean and Japanese sentence enders is amusing, hah. 네 is like よ (and not ね), 요 adds politeness (yet hearing ㄹ까요 sounds like a mild attack if your head is in Japanese mode). What fun. :P
雲 は きれい だ よ。
So there is the exclamation mark at the end of the sentence.
'clouds are really beautiful' was not accepted
'-네' is a declarative sentence
the clouds sure are beautiful!