Yeah in my opinion I don't think the movies managed to properly convey the "ancient-ness" and the "cosmic-horror" ness of IT. This is a creature that is the enemy of a being that created the universe, a being that has existed since the beginning of time ; and yet in these movies IT seems so... mundane, like something that is just as powerful as a random monster in a random horror movie.
I haven't read the book, so maybe it's different there, but I always figured this was the irony of it and kind of a note for how pathetic he really is.
Forget about the idea that he's as old as the universe and just take his abilities at face value: He is a shapeshifting, mindreading, reality morphing, nigh invulnerable force of terror. So long as no one ever figures out that his one weakness is belief, he is completely unstoppable. The only reason the Losers figure that out is because of the sheer number of prolonged engagements they had with It that unveil this one weakness.
He's as omnipotent as you can be in the world while still having a vulnerability and what does he do? He spends his time scaring and killing kids. He lives in a small fuckoff nowhere town and the only people he chooses to exercise his vast, near unlimited power against are the weakest and most vulnerable of human society. Then once he finishes his binge eating he sleeps for nearly 3 decades before getting up to eat again.
Some of that is strategic, like targeting children makes sense because they won't be believed if they talk about him, but otherwise,It comes off as not just a bully, which is obviously what the story is an extended allegory for, yes, but also a....well, loser. Not living up to his potential in any sense of the word, picking on the smallest possible fish despite his great power. He's not a vast, unknowable creature like the cosmic horrors of Lovecrafts ilk whose motivations are unknowable and their regard of humanity indifferent. He's just a sadistic asshole who only wants to target people that have no chance against him.
In a way, it seems like almost a deliberate subversion of what you'd expect a cosmic horror to actually be. All that eldritch, worldchanging power, and all It amounts to is a child predator in some rinky dink town. Like you can imagine his other eldritch family or friends meeting up with him and them having driven a planet insane or re-written laws of physics in a part of the cosmos, and he has to admit that all he spends his time doing is sleeping and eating kids, and they're like "What happened to you, man?"