It's downvoted because Biden is honest about his problems with his speech impediment. There's no "haha gotcha" to be had from making fun of a person who openly has a disability. If the president were someone in a wheelchair, would you make fun of them for not being able to walk?

Trump is a valid target for mockery because he refuses to admit he has a problem, and he's built his brand off mocking marginalised people including the disabled. This is picking on a bully. Also Trump doesn't have a speech impediment, he has a thought impediment, and thought is actually necessary for a president, which is what he's trying to be. Biden already stepped down from the race.

Everyone will clap if you make fun of a bully who's trying to regain the power he can use to bully more people. Not many people will clap when you make fun of a person who identifies as disabled and already plans to quit his job due to it.