Trumps approval rating now isn't much different from his approval rating when he was elected. He's not likeable. He says stupid s***. But his lack of likeability doesn't make him a terrible president. Actually as unlikable as he is, his policies have been quite successful. The economy is stronger than it's been in years, unemployment is at lows we haven't seen in a long time. He's made progress with North Korea. He's secured better trade deals for the US with multiple countries. Both democrats and republicans have talked about improving border security for years yet done nothing. Trump came in and secured funding for it quickly. We can debate whether his plan is good or not (I don't think it is) but he at least tried to actually do something.
In previous elections the american people have focused more on results than likeability. Obama wasn't well liked and got re-elected. People had the perception that Bush was a dumb hillbilly but he got re-elected. Reagan had low approval ratings and got re-elected.
Also approval rating doesn't tell us how strong the support base is. Trump supporters are a passionate group. You see it in the size of his rallies. He may only have 41-45% approval but 35% of that could be tbat rabid base. No democrat nominee has that type of support.
Yeah Hillary was unlikable. But this group of democrat nominees is too lol. Biden has been embroiled in multiple scandals and despite being the former vice president he can't seem to pull away in the polls from people like Warren who is right up there with Clinton in terms of unlikability. That Warren is a top candidate is telling of how bad their candidates are. She's lied about several things in her past including her self proclaimed native american heritage. She can't explain any of her policies. Refuses to admit her healthcare plan would definitely cause tax hikes to the middle class. Bernie is likeable to his base. And his base is passionate. But his base is too small. And his campaign doesn't have the same oomph it had 4 years ago. Bloomberg is disliked in his own city.
I really think Buttigieg has a shot if he can gather support. He grew up in the Rust Belt. He's won tough races there. He would do well in key states like Ohio, Michigan, Wisconsin, Kentucky, Pennsylvania. He would appeal to younger voters. He would appeal to independent/undecided voters that are turned off by the far left policies of a Warren or Sanders. He would appeal to the independent/undecided voters who don't want to vote for someone in the old school political establishment (big reason why Trump won in 2016, people are tired of these career politicians and are looking for someone to shake things up).
I can't see Biden, Warren or Sanders having a chance. I don't think any of them having a big enough or passionate enough base.