I think its high time that the Straw Hats get a new nakama. And Monet seems to fit the bill for Straw Hat weirdness. Also the fact the Doflamingo threatened to kill her if anything happens to Caesar Clown. This might be what makes her join the crew.
She might reveal how afraid of Doflamingo she is and given how Luffy doesn't like people who threaten there own nakama vows to defeat Doflamingo. Monet out of gratefulness might join believing Luffy could protect her and ultimately free her from Doflamingo.