Fighting against Thanos should equal tons of growth. The guy had the power to kill half of life in the universe. This movie went backwards with the character. He already went through growing pains in Home Coming. Spider-Man 2 had him in college and a recognized hero.
What growing pains did he have in Homecoming that had him dealing w/ essentially being dead for 5 years, having his mentor die to save all of humanity, then having folks expect you to fill those same shoes at just 16.
If he still mopping around bout Tony and heistant bout being a leader in Spidey 3/4. Then you can complain about his lack of growth.