Eh, there's enough good options to get you through most of the game on only Johto Pokémon. Ampharos is good, Heracross annihilates 5 gyms for no reason, and Espeon puts in work at points (so I've heard, I haven't actually used this one). It's very obvious that they wanted teams to mix Kanto and Johto Pokémon, though. The red Gyarados in particular gets a DDance setup on literally every member of the Indigo League if you can get it to level 44 (though you may not want to spend too much time setting up on Karen, that Umbreon has some infuriating RNG abuse if you give it the necessary turns), and that's given to you for free. It does make Johto feel kinda lame when so few of its Pokémon have the necessary combination of strength and availability to be good in in-game playthroughs, though. Those three I just mentioned? Those are some of your only good options. Have fun!
It's especially frustrating in HG/SS because a bunch of Johto Pokémon got really sick evolutions that you can't use for most of a playthrough for no real discernable reason. Togetic can't be evolved until the post-game, you can't get Magnezone at all because no magnetic field, Weavile and Gliscor require you to play the fucking Battle Frontier to get their evolution items (hope you like the Battle Factory, because you probably don't have good enough Pokémon to win the necessary BP in anything else if you're just doing a casual run), and a bunch of other cool ones are locked behind trades that many people can't do legitimately due to a lack of the necessary equipment/friends. These Pokémon would have made HG/SS playthroughs way more interesting, but I guess not.
What a frustrating game.