That sentence means "Raining is not (also) better at all" "Trời mưa" is "Raining". It's subject-verb clause, play role of main subject in sentence. "cũng" indicates repetition of result, another example is "Dù chuyện gì xảy ra, tôi cũng ở bên bạn", means "Whatever happens, I will stay in your side" "hay gì hơn đâu", it's just kind of thinking/judgement expression. It has same meaning of "hay hơn đâu", "tốt hơn đâu", "tốt gì hơn đâu" or "better". The word "đâu" at the end of sentence is used to modify or stress the negative meaning.

It doesn't get better if it rains.