What do you mean by ruined? Do you mean how Ash beat every gym on his first try except for the first and last, or their overall battle styles?
Pretty much how Ash managed to beat every gym on his first try except for Brawley (probably his only REAL loss in the region. I'm not counting the earlier battle with Norman as that was more of an excuse to force Ash to go the route of the games.), and with a purely all-rookie team no less. At least Johto had him using some of his veterans besides Pikachu. The only Hoenn gym leaders that genuinely impressed me there were Brawley and Roxanne (the former for, you know, actually beating Ash, and the latter for the fact that she at least to Pikachu, a confirmed veteran). Plus, to a certain extent, there's the battle styles. Aside from the infamous Tate & Lisa battle victory as CodeHelmet referenced earlier, there's also the time where Ash and Pikachu fought against Wattson. Technically, Wattson did at least lose to a veteran as well, but the problem was that both Ash and Wattson ended up technically cheating in the match (Ash by doping Pikachu by accident by taking out the Robotic Raikou, and Wattson by even using the Robotic Raikou at ALL), which STILL made him look very bad and incompetent. To sum it all up, it's hard to take the Gym Leaders all that seriously as being cr?me of the crop trainers that the OS alluded to when they lose as easily to rookies as Jessie James and Meowth are known to do (including especially the third episode where they infamously lost to a near-dead Caterpie despite having literally every advantage, and that resulted in them gaining a reputation among both fans AND certain in-universe characters as being trainers who were so pathetic that beating them at all is not even a brag-worthy achievement), which is what Hoenn did with them.
Sinnoh, I'll admit did better in that the Gym Leaders actually did manage to beat Ash more often than not, with Ash actually having to struggle to fight them with his team. Unfortunately, it still didn't quite work since, thanks to the infamous level resets that Ash and Pikachu had to go through, it came at the cost of Ash and Pikachu looking incompetent where they lose despite them beating a Regice single-handedly just a few episodes back, which STILL makes the Gym Leaders look pathetic in another sense.
Unova arguably was even WORSE in its depiction of the gym leaders than Hoenn was. Aside from it repeating the exact same mistakes Hoenn made with the gym leaders by having them lose easily to Rookie Pok?mon, thanks to how Ash was infamously characterized in that saga, it also came across as them losing to someone who was brain damaged.
Kalos was, until the Kanto Reunion arc of Alola obviously, the closest any of the anime series came starting with AG earlier to EVER redeeming the gym leaders by depicting them as truly skilled trainers as their reputation claimed in the OS, where they actually managed to beat Ash multiple times in a way that DIDN'T make Ash look incompetent as a result. Even the one win Ash DID get that didn't require a rematch beforehand, it was made explicit that the gym leader in question used a self-imposed handicap. And we actually SEE what the Gym Leaders do so we get a gauge of their true strength beforehand.