비가 오려면 오든지, 맑으려면 맑든지, 날씨가 잔뜩 흐려서 이게 뭐야.
=> If it's to rain, let it rain. If it's to clear up, let it do so. What's (with) this massively overcast sky?
=> If you're going to rain, go ahead and rain. If you're going to clear up, do so, instead of giving me this massively overcast sky.
So it's a complaint about the overcast weather.
* ~려면 ~(하)다 is a simpler (and more impersonal) way to say ~고 싶으면 ~(하)다. (가고 싶으면 가 = 가려면 가)
* 이게 뭐야 is literally "What is this?", but the implication is "I don't like it", "How awful", etc.
* 잔뜩 흐려서 이게 뭐야 = 잔뜩 흐리고 이게 뭐야 = (It's) Heavily overcast and (as a result) I don't like it.