"Rainy days and Mondays always get me down."
I wouldn't use "more rainy", tho perhaps some people would. Most commonly, neither "rainier" nor "more rainy" are used to describe wet weather. We would say, for example, "June was a wetter month than May" or "We got more rain in June than we got in May."A teacher asked me today: "Do we say rainier, or more rainy?; snowier or more snowy?"