Candidates usually present themselves as more extreme in primaries and more moderate in the general election. Says something that Democratic primary voters *wanted* the more moderate candidate. I think people really miscalculated on 2016. In the primary, Clinton was largely unopposed. Because of that, all the opposition to Clinton within the party for any reason consolidated around Bernie, which confused a lot of people into thinking that there was a really strong far-left movement. Then there was a popular conspiracy theory that the race was stolen from Bernie, even though it was never close. The result of those two things is that people thought that the primary electorate was much further left than it actually was. Then in the general, there was a perception that Trump was a joke candidate and extreme right, and his winning convinced people either that he was a political savant or Democrats were unhappy with Clinton being too moderate and didn't show. In fact it was a favorable year for Republicans, and a better candidate would have done better (remember that Trump lost the popular vote), though Trump's (actually moderate on economics) message was optimized toward uneducated voters, which gave him a better electoral map. Those mistakes led a lot of Democrats to think that the key to winning was to run hard left. But Biden's success, both electorally and in office, is leading people the other way. Moderate candidate crushed, and then he's actually gotten a lot done in office. The key to moving forward on policy is not extremism but competence, and Biden is showing that.
I'll ignore the error and the childish personal attack and just point out that Trump has long been known to be an extremely crooked businessman, while Biden is as certified non-corrupt as it gets. Dude was in gov't forever, and quite powerful, and was still broke (poorest Senator) until he retired and signed a big book deal, and did some speaking. Of course Republicans are going to say whoever Democrats nominate is the Worst, Most Corrupt Human Ever, but looking at it reasonably, it's an insane claim.
49% according to Rasmussen, which is crazy high these days. But instead of cherry-picking, it's better to look at an average, and right now it's about 42%. But that's general approval, which is influenced mainly by partisanship and the state of the economy. Other than Republicans who just think every Democrat is an evil monster, there aren't many who think Biden is more corrupt than Trump, as that belief is just completely bonkers. Similarly, it's very obvious who is more respected internationally. There's a reason that enemies of the U.S. love Trump so much.