I don't think Luffy has a huge attachment to his mother, for all intents and purposes, Dadan filled that role in his life and she fits the descriptions Oda gave of his mother.
Obviously someday we're going to get Dragon's flashback and we'll meet her then, but for how much she actually affects Luffy? That's hard to tell.
Just look at all the new depth and background we've almost out of of nowhere started getting about Sanji. He's been an orphan as far as we've known the entire series, and now instead he has a prestigious family and lots of brothers and sisters. He's never brought it up before now, because it wasn't important. But hey, hidden depth!
I didn't necessarily mean attachment to her. But if she had any kind of role in his life after giving birth to him, she might have imprinted something on him. An idea or some kind of longing that managed to get stuck in his mind for all these years. Something in that sense.