I think it means 'You are not worthy of Yukino-san'
Is this correct?
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「ゆきのさんにはもったいない」 would be "(Someone/something) is too good for Yukino-san" → "Yukino-san is not worthy (of someone/something)".
"You are not worthy of Yukino-san" would be 「ゆきのさんはあなたにはもったいない」, "Yukino-san is too good for you."