It just like how you understand this sentense. you can cut this sentense into two part. 1."天/真好"means what a good day! 2."天真/好" means to be innocent is good. it is up to you. but if you put it into certain situation, the chinese people will know your exact meaning.

1. If you want to strike up a conversation with sb. ,it is a good choice to see "天真好" or “今天天气真好”. 2. Maybe when both sides have nothing to see, they will take this topic to get both of them free of embarrassed.

If someone say: 天真好. It means he want to talk with you maybe,lol

it means what a nice day. and many people use it as a way to great each other:)

it means that it's a fine day