Translation:Isn't the wind weak today?
Its not wrong, DuoLingo is wrong in using "isn't". You are literally asking "is the wind today?" Answering "yes" means you think the wind is strong. However, if you asked "isn't the wind weak today" as DL (incorrectly translates it), and you said "yes", then you mean that the wind is weak.
The way DL phrases these is incorrect according to multiple, albiet non-native, Japanese speakers. If I wanted to say "isn't the wind weak today", it would be 今日は風が弱いですね" where "yes" means that it is weak". By phrasing it with 弱くない, you are literally asking if its "not weak". Saying "yes" means you think its "not weak".
I just keep reporting these since DL needs to change the translation to "is the wind not weak today?" Although it sounds a little weird like that in English, its not technically wrong, and its a more accurate translation, as a "yes" response to the japanese question or english question would both imply that the wind is weak (or at least not strong).