"Isn't the wind weak today?"
The sentence topic is about the wind, specifically today, but the subject of the wind is whats acquiring the adjective. So in order to have the adjective not floating on its own, it seems to be more preferred to use が to indicate the subject being described rather than は as a topic indicator in this instance.
I think this sentence in English has the opposite meaning against the one in Japanese. "Isn't the wind weak today?" sounds to me like "The wind is weak today, isn't it?" While in the Japanese, people tend to use negative question to express the affirmative sense in a humble and objective manner. So 今日はかぜがよわくないですか actually means "The wind isn't weak today, is it?"
Shouldn't 「今日はかぜがよわいだね。」 be be accepted? It would, more literally, translate as "The wind's weak today, right?", which means the same as "Isn't the wind weak today?", doesn't it?
EDIT second time I run into this question, now I got 「今日はかぜがよわくないだか。」 and it marked 「だ」 wrong, should've been 「です」 instead... I... What?
よわい (weak), and also よわくない (not weak), is an い-adjective, which do not use だ afterward. Only the other type of adjectives, な-adjectives, use だ afterward in the informal form. For the informal form of い-adjectives, you do not have to add anything afterward to say "is", even though the polite form has です afterward. If you wanted to say this sentence in an informal way, you could just say 今日はかぜがよわくないか。
Why is 風は今日弱くないですか (Kaze wa kyou yowakunai desu ka) wrong/not accepted? I have done the same thing many times in other similar sentences where usually they put the adverb of time at the beginning, sometimes even without a topic particle, and it was always correct. Why is it different in this case? Could it be because this is an interrogative sentence?