The point of Taxi Driver isn’t that society is bad, and things just suck. It’s much more a commentary on the choices Travis Bickle have made and the person he is. Ironically, much like what you’re suggesting, things aren’t bad for Travis because society is harsh and people are mean to him. Things are bad for Travis because he’s a mentally and emotionally unwell person who instead of working on himself obsesses on the negatives of his life and the surrounding world… but is unwilling or unable to make real changes to his lifestyle or attitude.

It’s important because the negative aspects of Travis are the negative aspects in most of us but taken to the extreme. A lot of us don’t take accountability for the state of our lives, for the decisions we’ve made, a lot of our displacing blame and don’t actively deal with our emotional turmoil. But instead displace it on the world around us. There’s a lot of anger in the world just like Travis’s who ultimately decides to channel his anger in an extreme act of random violence which is also unfortunately how a lot of people channel their turmoil in this country and world too. It’s a character study that has only gotten more relevant tbh.

So yeah, if anything Taxi Driver is In itself a criticism of the negativity and cynicism you’re describing but it criticizes those things by exploring a deeply flawed protagonist who brings us into his deeply flawed world view by the very design of the story. It criticizes it by portraying it.

If you just wanna watch happy movies that make you feel good about how things are that’s fine. But don’t try to portray it as like noble or something or like what the world needs more of. There are plenty of mindless entertaining movies that people love and are available if that’s your thing. If anything there are less and less movies that aren’t just mindless entertainment around. And if those are what appeal to you that’s valid but it almost verges on fascist to be like: “yo let’s not have negative portrayal of how things are if they bum people out too much. Let’s instead disingenuously make art about how everything is golden and if you try hard enough all your wildest dreams will come true.” Like that’s not the point of storytelling or art.