。
Haruna's mood is completely dampening Kaoru's spirits.
Literal
Completely Haruna [gen-の] weather-mode [gen-の] [thanks-to-おかげで] Kaoru [gen-の] fighting-spirit [subj-が] is-being-dulled [emph-な].
お天気モード 'weather mode' extends the 'weather = mood' metaphor (お天気 commonly stands in for 'mood / disposition' in Japanese) by combining it with モード (mode) to make a playful character-describing phrase. おかげで 'thanks to X' is the causal/benefactive marker, often ironic when the cause is negative — here clearly sarcastic. 気勢を殺ぐ is a set idiom 'to dampen spirits / take the wind out of someone's sails,' where 殺ぐ (そぐ) means 'to dull / chip away.' The passive 殺がれている is the resulting state.