No point in denying that Hillary Clinton is polarizing and has high negatives. But I think the constant commentary on her being less popular than other notable Democrats usually comes without sufficient context. Let me go through the other names on this list.
Michelle Obama - Of anyone who can currently be called a household name in US politics, there's hands down no one more awesome than Michelle. I'm a big fan of Barack, and Michelle has never shown any interest in running for office, but I'd take Michelle running the country over Barack for real. She would truly be the sensible non-politician this country needs and the women does not suffer ribald imbeciles.
Bernie Sanders - He had an electrifying movement that captured the hopes and dreams of many Americans who didn't even realize they were social democrats until Sanders took the simple step of calling himself a socialist apologetically. He'd surely have higher negatives if he had won the nomination, but because he's only a what-if "one that got away" former-hopeful now, people tend to be have a positive slant when thinking back about him.
Bill Clinton - the fact that he hasn't run for anything in 20 years certainly helps boost good will for him. But if I was ranking the speakers, I'd choose Bill Clinton, easily, as my least favorite. While Hillary acknowledges that some of her past policy decisions were bad and explains how she plans to implement better, revised policies going forward, Bill is way too obsessed with his own legacy. Whenever Bill's record is questioned, he just disregards the fact that he's supposed to be helping Hillary win. Instead loses his temper at grass roots leftists and random college kids, bombarding them with self-congratulatory defenses of his own presidency. His behavior played a not-insignificant part in Clinton's 08 downfall to Obama. Bill Clinton is selfish, narcissistic, and an egomaniac. I will never understand this guy's appeal, and believe Hillary is the more impressive one in that couple by a mile. But he's very popular.
President Obama: He pulled the upset win against Clinton and took the nomination for a reason: He's damn good. He checks every box. He's young (for a big time politician), he's incredibly intelligent, he has an extremely pleasant persona -- politics notwithstanding -- he seems like the kind of person you'd love to have as a next door neighbor. And he's also just really cool. Obama is more popular than Clinton on his own merit.
Biden: Everyone loves this guy, but again, the grass is always green on the other side. People aren't happy about the two choices they're left with, and everyone who stayed out of the race suddenly looks appealing. Let's also call this the "Eagles Back QB Effect". Eagles fans are notorious for their unwarranted optimism and trust for the team's back up QG. When the starter has even a bad half of performance, many start howling that it's time to give the back up QB a chance. Of course, we usually hate the back up even more if we get what we wished for. Bottom line is that Biden has a great personality and also has the advantage of being the backup QB who never actually took any snaps.
Chelsea: She's pretty non-descript and is much more about supporting her mother than pushing any political agenda. I don't know why anyone would dislike her, but she's also not particularly good at public speaking. I'l be interested to see why they broke tradition of the nominee speaking last in favor of giving Chelse an epilogue role.
Point is that Dems have some damn good people, but most of them are done with running for office. Clinton still is running, and it depresses her numbers, just as it would if Biden or Sanders were on the ticket.