In general, yes, run a candidate that is popular with voters. You can’t do that this year. No one who is unknown would have the time to get enough traction to beat Trump. No one who is well known is liked enough to motivate people to get to the polls.
I don’t think people are saying don’t criticize Biden or don’t criticize the system that resulted in Biden being president the first time and a candidate the second time. By all means, criticize away. He could be worse, sure, but he could be so much better.
What I think people are saying is that when you are comparing Biden and Trump, it’s important to point out that while both are not ideal, one is way worse. People who aren’t paying attention to the level they should will be less motivated to show up to the polls if they just think “everybody sucks.” Biden isn’t great, but Trump is a criminal scumbag who wants to let others strip us of our rights so that he can play king.