As you say. I thought the movie got so much praise because of the experimental animation, artwork, vibrant supporting cast, and cohesive world-building.
You'll have a hard time convincing me that the movie would have been hype if it had been forced to rely on Mile's story of the power of believing in yourself mixed with a fun uncle vs. responsibly daddy plot.
I hear what you're saying. The jump from kid who doesn't know what he's doing to a sudden badass out Spider-Manning Spider-Man seems silly.
But if there was a gradual buildup of Miles becoming Spider-Man, it would go against the entire point of the movie.
He was a victim of circumstance and got bitten by a radioactive spider. Like he says at the end, anyone can be Spider-Man. And when you see the Raimi films, it's the same thing. Parker got bitten and could suddenly do crazy gymnastics and beat up the bully and eventually the villains.
This isn't a case of someone training or working hard. It's about accepting the responsibility that comes with the new powers bestowed upon him. Up until that point in the movie, Miles simply didn't believe in himself, but he had all that power the moment he got bitten by a radioactive spider. The tipping point was his interactions with his family, especially after his uncle's death (an obvious Uncle Ben parallel that was stated by the 5 other dimensional Spider, uh, people).
Up to that point, I still believe Miles was a sympathetic character, if flawed. He was going through things high schoolers go through (the need to be accepted, embarrassing situations, adapting to a new school, etc.) and his story offered a fresh version of the high school Spider-Man story we get rehashed with every reboot.
Now, would the movie be as strong as it was on the strength of Miles' character alone? Probably not. But I don't think he hurt it. He's taken the mantle and the series can go in a nice new direction alongside the other Spider-Man movies. I can't say the same thing for the MCU reboot or the duology after the Raimi films.