Obama left office at +22 approval in Gallup (59 / 37). With a 90+% approval amongst Democrats. He isn't polarizing to Democrats who can be swayed away from Bernie.
But, yes, people love Michelle.
The dems don't need to sway other dems, they need to sway other independents, and also, to convince the women to come out and vote that stayed home last time because of their dislike for Hilary. They don't seem to realize that fact though, hence the support for Bernie.