Poor strategy killed her campaign. Poor tactics doubled down her poor strategy. Poor decisions (remember when she decided to self host email), poor judgement when in power followed her entire public life. Look for a demoralizing 2024 run if Biden doesn't run or crown someone.
In the 2016 race, the Clinton campaign spent about twice as much per popular vote received than the Trump campaign. So by that metric, Democrat messaging wasn't successful.
Well, no, they have numerous objectives, many of which are in tension. In rough order of priority for a typical Presidential general election campaign, though there is considerable variation from campaign to campaign:
(1) Maximizing the probability of at least 270 EC votes (this can be quite different, and conflict with, maximizing the expected total number of EC votes), because that’s how you win,
(2) Maximizing downballot coattails, because that’s how you get people in position to pass your agenda into place, and get the people that are in that position to see you as important to their position,
(3) Maximizing EC and Popular vote totals, both of which are important to the perception of a mandate, which helps your agenda.
I mean, Trump won with several million fewer votes than Clinton. The common narrative around Hillary Clinton and the Democrats is, as you mention, the failure to tap into an anti-establishment zeitgeist that Trump did more successfully. But the numbers alone (apart from electoral votes, obviously) show the Democrats were at least as successful, if not technically more successful, than Trump at messaging. The Democrats' failure was one of tactics, not strategy.