I keep hearing that the Democratic Party should've paid more attention to the young white American demographic. Does this mean there was a point to the "All lives matter." movement?

The left has been openly hostile towards white people, and especially towards white males for years now. They've accused every single one of them of being racist just based on the color of the skin they were born with. They've said its impossible for them to experience racism themselves while being racist towards them in the most literal sense. Liberal state California tried and barely failed to pass a law to make racial discrimination legal. Liberal School UC Berkeley banned white people from the community gardens on certain days. Liberal companies like Google have purged white & Asian males from their applicants list. DeAnza college was illegally targeted white people of both genders. The list of offenses goes on, and on, and on. They've told white people and especially white males that their voices don't matter and they need to shut up and get out of the way.

The Democrats who support this type of perspective shouldn't be terribly surprised then that the demographic they've been vilifying didn't come out in droves to vote for them. Many didn't vote for trump, they just didn't vote at all. This comment will likely get downvoted all to hell, I'll be called a racist without anyone even knowing my race, and a number of other things. This is a conversation that most people on the left refuse to have, even with other leftists, Democrats, or allies, unless the conversation is supportive of the ideas.

But to answer your actual question, not really. All lives do matter, but the All Lives Matter statement was made mostly by people who didn't understand the BLM movement, or didn't understand that the name doesn't imply that other lives don't matter. The best explanation I've read is that if someone's house is on fire and they call for help, you don't respond by saying "well all houses matter". You put out the damned fire.