My personal opinion is that Clinton should have reached out earlier and emphasized core Party issues important to Millennials, including an increase of the minimum wage to $12-15/hr, affordable college education with a focus on her community college program, as well as affirming her support for climate change legislation with a free market incentive based model.
We'll have to wait a few weeks before we can conclusively say anything about this. I totally agree with you, though. She seems to have acted like Millennials would end up voting for her regardless and essentially took them for granted, and look at it now. She is only truly beginning outreach, and it will take time.
However, and this is sort of unrelated but I'm going to say it again anyway, I can't really emphasize this enough: Unless Clinton somehow manages to deliver on policy for Millennials and find a way to show them she isn't nearly as bad as they think, she is going to be a net negative for Democrats and their support among Millennials. The worst thing our party could do when Millennials are an important part our of base is to nominate someone as reviled by them as Clinton is. If she remains unpopular and has a bad presidency, she could drive young Millennials who are not fully committed to the Democratic party away. The 18 - 25 year period is where young voters are most impressionable and liable to fluctuate in their leanings.
We're just lucky Republicans nominated Donald Trump, or else they might have had an opening to weaken our support long-term among that group.