There is nothing wrong with the johto level curve. The levels being lower than normal isn’t a flaw because you just naturally gain less EXP throughout the game, it’s different from gen 1 and focuses more on other aspects instead of spamming immense amounts of trainer battles everywhere and that’s ok.
The non linear sections after morty have trainers that are leveled such that those you are likely to fight first (out in the open on routes primarily) are around 16-20 for the ones you can fight before morty and 18-24 for the ones you can’t fight till after morty. This is pretty normal, and you’re not gonna be anymore than 5 levels higher than that if you’re playing normally. They’re on the easier side, sure, but they’re supposed to be.
The tougher battles that stand out more are hidden away, like the trainers in the secret spot on route 34, or the ones in the Lower levels of union cave. If you don’t battle any of these trainers you’ll be rather underleveled for the gyms, likely around level 25, this is intentional. You can still beat the gym leaders at that level if you’re an adult, but if you’re a kid it’s gonna be like hitting a wall, and it’s designed this way to incentivize exploration. Exploring not only lets you find the highest leveled Trainers, but also the highest leveled Pokémon. Good sources of EXP are a reward in this game. Basically the whole latter half of johto is designed this way: common trainers on routes are around 5 levels below you, tougher trainers are around your level, gym leaders are about 5 higher. It’s appropriate game design. Weak ass trainers exist to make the strong ones stand out.

Tldr it’s not perfect but it’s designed with a purpose and doesn’t ruin or even significantly hinder the experience. I never see anyone complain about fighting dozens of level 25-33 trainers before gyms 10+ levels higher in gen 1. Fuck this shitty meme complaint