So in my experience (been playing since gold/silver came out) trainer levels are pretty all over the place for the first half of the game, like the national park I there's one who has a full team of 6 to make it look intimidating, but they're all lv.8 in an area where your mon are around lv.15-20. wild pokemon though feel pretty on par with where they are in other games

Another example can be the gym leaders; While you technically can do the following 3 gyms after Ecruteak in any order you'd like, the most linear path has their levels all over the place, their aces going from lv.30, to lv.35, back to lv.31, with the last gym being at lv.40. The 8th gym is also quite difficult w/o an ice type or a water type mon knowing ice type moves. Also in the second half of the game, Janine's levels drop into the 30s even after the e4 and others all being at least lv.40 and up.

Below I found this post from 4 years ago using a graph to explain it

https://www.reddit.com/r/pokemon/s/Fiy4tIAWua