The sheer number of games had to have taken a toll - out of the 63 games theoretically possible to play, Liverpool played 63 this season. Still only lost four!
As for it being a bit of a letdown only winning the two domestic cups after the potential of four trophies, sure, but not as much as could be. Liverpool, and its supporters, tend to be pretty cheerful regardless of circumstance anyway. Plus there'd been a bit of a trophy drought recently - until the 2018/19 Champions League, it was just the league cup in 2011/12 since... 2006? So that puts it in perspective.
And generally, I think Liverpool supporters are just loving it at the moment. Winning the quad would have been ridiculous. But just being that good to make it that far and winning a double? Still fantastic. There was a parade on Sunday (for the two cups, and for the Championship for the Women's team, but also a bit for the league win in 2019/20 when a parade wasn't possible), and you wouldn't think it was for a team that lost one final the night before (
https://www.liverpoolfc.com/news/pictures-liverpools-end-season-city-parade).
That said, the final match itself was a bit of a weird one, for reasons outside the game. I used to go to a fair few games at Anfield before our first child was born, and I still follow a fair few other supporters on Twitter, and some other LFC and football related accounts. So in the build up to the game, I was seeing a lot of tweets about crowd organisation problems, turnstiles not being opened, supporters being tunneled through narrow passages, people with legitimate tickets not being let in, people being mugged, then tear gas and pepper spray being used on crowds including children. So that was a real damper on the mood for the game itself. Then afterwards, more accounts of assaults and muggings.
By all accounts (including general media ones, not just LFC ones), with the exception of certain French authorities who are blatantly trying to cover their own arses, the organisation was a shambles that could have been a disaster. A few examples of many:
This guy had his ticket and phone stolen on the way to the stadium:
The Mayor of Liverpool was also robbed:
The wife and son of Jason McAteer (ex LFC player) were assaulted and robbed:
The game itself was basically Liverpool vs Courtois, with Courtois winning of course. And speaking of class from Courtois, with regard to the above, I thought this was a nice touch: