It's not that difficult, but it depends how much time you want to spend on it.
Some people are very overleveled because they take time to train, other people just want to beat the game, and tend to have a harder time, cause of lower level pokemon.
The idea of pokemon games has also changed in the years I think, people who started playing pokemon Red/Blue/... are now much older, and still play, still enjoying the playthrough, but also looking for more.
Best movesets/challenges, and make the game harder for themselves.