___ばよかった is basically "should have" expressing regret for not doing what would have brought different outcomes. ・早く寝ればよかった。 I should have slept earlier. ・コンビニに寄ればよかった　I should have stopped by a convenience store. ・最後の１個、買えばよかった。I should have bought that last one. So I would translate 夢ならばどれほどよかったでしょう as "It would have been better if it was just a dream."

Hi. I am not sure what kind of answer you are expecting. However, the phrase you refer to is probably belonging to so-called "3rd conditional". I 'll show you my translation for example, "夢ならばどれほどよかったでしょう" = How good would it have been if it had been just a dream. "夢ならば” suggests "something not a dream and I don't want it " "良かったでしょう”suggests "actually not good to me at the moment". Although My English is not really good to tell you all about Japanese. I hope my answer would solve your question.

Olá?