That's the main problem with the Democrats right now, they've forgotten to play the ball and not the man.
The Conservatives learned this at their expense in the last election with Corbyn. When your opponent is such a ridiculous choice as leader, when they're an extremist in many of their views, and are just thoroughly unfit for politics, attacking that just plays into their "anyone but the establishment" rhetoric. It makes you look like you're attacking them because they're not a part of the establishment rather than because they'd be a complete fudging horrorshow if they ever got into power.
If the Democrats stuck with a message of trade tariffs are bad, protectionism is bad, removing people's human rights is bad, etc they'd be doing a lot better. Unfortunately, laughing at Trump because he talks bad and thinks bad just makes them sound like snobby Californians laughing at the rednecks.