Democrats have traditionally been worse at messaging, agreed. But there's a new crop of catchy slogans that seem quite popular at least among a loud subset of Democrats and which can't really be explained by "terrible messaging". It might be explained by choosing the wrong audience.
Take, for instance, the infamous "basket of deplorables" comment. This wasn't hard to understand. Nobody needed to explain anything. It was just offensive to anyone who had doubts about parts of the Democratic platform in 2016. This isn't about bad messaging, this is about bad intent. "You're not with me, so I'm gonna give you 50-50 odds of being racist, sexist, homophobic, xenophobic, and Islamophobic. Now, how 'bout you vote for me?" This plays really well with the hardcore supporters who want to belittle the opposition. It doesn't play very well with swing voters. Simple message, effective message, wrong target audience.
Take, for instance, McAuliffe's election-losing statement, "I don’t think parents should be telling schools what they should teach." This isn't hard to understand: the board of education and teachers decide what your kids learn, and you don't. The problem is that a lot of parents care about what their kids learn--the message "shut up, go away, we've got this" instead of "we want to hear parents' concerns and balance all the different perspectives" isn't an issue of needing to explain. The attitude is offensive if you're worried, which you're more likely to be if you're a swing voter. But it plays well with the hardcore supporters who just want the conservative nuts to quit trying to take books out of schools and pretend homosexuality doesn't exist. Simple message, effective message, wrong target audience.
"Defund the police" you would think is a classic example of bad messaging in that supposedly this means to spend more on non-police social support services so the police can concentrate on tasks you actually need armed police for, thereby using your budget more efficiently. But a bunch of cities--almost all Democratic ones--simply reduced their police budgets without first establishing a credible alternative social support service. So either the messaging was so bad that even the supporters lost track of what they were asking for, or in fact the message wasn't as complicated as it seems. "Police bad. Police racist. Police get less money." That's what it sounds like the slogan means, and that's mostly how it worked out in practice, and that plays well to a certain crowd (especially when they get to feel all smug about knowing that it isn't supposed to mean what it says--the interesting part is in what isn't said). But not, for the most part, the crowd you need to sway to win elections. Simple message, effective message, wrong target audience.
One could go on, but no, I don't think the hypothesis that it's simply bad messaging explains it.
(Note--Republicans have the same problem on their side. I'm just pushing back on the idea that Democrats' problem these days is just messaging rather than the actual meaning.)