People dont really bring Misty up as this powerhouse trainer because she was was just never remotely impressive. Ironically all her most impressive battles were against Ash as his worst as a trainer, and even then she lost even with the plot giving her benifits (pikachu refusing to battle, poliwag evolving etc.) aside from the whirl cup, where she was on her home field, and still only managed to beat Ash's weakest pokemon EVER, before needing psyduck hax and Ash going derp mode.
Depends on what you define to be impressive.
It can be number of evolutions someone has in pokemon roaster, battle record. Type of tournaments or competitions spmepne entered and scored good result or type of trainer he/she batled against.
People often forgett how Original series came from 90's era where CGI effects wrent even aplied yet, animation was on lower level of quality, movepool of pokemon species was more limited in earlier generation games and writers werent as experienced in depicting battles in same detailed, diverse way like it became standard later on.
But is it fair to take this deficiencies as argument of Misty battle moments being less notable and valuable than its situation with new characters who were introduced in era where visual effects and style of writing when it comes to pokemon fighting was more flashy and eye pleasant?
Not, not really becasuse for back in day standards Misty wasnt portrayed as any less skilled, intelligent or innovative trainer than successors were if you ask me.
For instance she showed high level of adaptability and knpowledge when winning Princess tournament going all way to the top with pokemon types she never used before. Such as Vulpix, Pikachu, Bulbasaur from Ash/Brock.
Yea, you can argue those werent her pokemon. But it was Misty who made calls and choice of techniques there, and when taken in account how she was solely water specialist not training or practicing with other pokemon, her command and choice of attacks with different types was still fairly remarkable.
Misty defeated several years in a row champion Andreas at Seaking tournament despite Poliwhirl being at severe disadvantage against fully evoled form Poliwrath.
She put under control enraged Gyarados earning his thrust much faster than Ash did with Charizard, Iris with Dragonite, Dawn with Mamoswine etc. With PIA inspector Joy recognizing her potential as trainer and great level of understanding in knowing how to establish close bond with water pokemon. This was displayed in strong friendshgip she established with Marill from "Crying out Loud!" as well.
We saw her coming top 4 in Pokemon Athon race from Kanto, won Balloon race, won Alto Mare race, defeated several much older trainers than her or left positive impression on them.
Such as Keita and Farfetched, Harrison and Quilfish, Trinity and her Gyarados, Dorian and Mantine, guy with Delcatty, colonel Hanson and his Shedinja who was no match for Ash, Brock, May pokemon back than etc.
But no one really remembers those with Whirl Cup being only thing people acknowledge. Simply because Misty moments where she shined as trainer or person in actively searching for solutions to fix problems happened more in standalone episodes or when dangers of some sort occured.
Rather than in contests or tournaments which will always come first to people mind over anything else.
Hence explaining popular misconception of Misty only real achievement beng Whirl Cup circulating among fandom(which is far from truth).
Her skills were viewed as above average by several pokemon trainers out there with strength of her pokemon and resistance leaving positive remark. Such as gym leader Koga and sister Aya being astonished by unusually high power her Psyduck posess never seeing such huge psychic energy from that specie.
Water trainer Marina noticing how well trained and strong Misty Staryu was when it defeated Tentacruel with only one attack.
PIA inspector Joy being impressed by Misty quick thinking, determination, resourcefulness and improvisation in dealing with pressured situations having required skills and strength to be gym leader.
Brock younger brother Forest is often brought up as"special kid" because of being gym leader at such young age, yet its overlooked that this applies to Misty as well with not many trainers proving themselves to be capable of holding this position already at age of 10.
People talk about this huge battle abilities Brock has when it comes to pokemon and im not dyenying that. But have they ever bothered to compare moments where Misty and Brock innovation and resourcefulness as trainers was put to test?
Such as in pokemon 3rd movie and "Spell of Unown!" where Brock was quickly overpowered with Molly mirage pokemon who were multiple times stronger than normal ones.
Yet in battle aganst Misty, her pokemon like Staryu werent overpowered but held their ground against Molly pokemon like Mantine being on even field . Never mind fact that Misty showed more advanced level of strategy there than Brock did giving Molly run for her money.
Speaking of Ash, by time they faced in Whirl Cup Ash wasnt anymore rookie, He reached top 16 in Indigo league which was higher than his rival Gary achieved. Conquered Orange Islands league becoming its champion, won various competitions like PK1 Grand Prix, Tauros battling competition, Bug catching contest, Sumo conference Match, ended as runner up in Grass pokemon tournament etc.
So he was definitely not at his worst when facing Misty in second round of Whirl Cup, not counting preliminary battles.
Meaning that if other main characters battles against Ash can be described as "shining moments" and indication of their talent, than Misty win should not be devalued either.
As for battle record, first battle at gym in "Waterflowers of Cerulean City" was tied,. Misty defeated Butterfree with Staryu, Ash was about to defeat Starmie with Pidgeotto. Match unded unfinished.
In battle for Totodile Ash won, but barely. With Bulbasaur being on his last legs with solar beam being its saving grace.
In Whirl Cup, Ash may had lost to Psyduck because of his reckless nature and obliviousness but is there any guarantee how without Psyduck Misty had no chance in defeating Kingler. Especially when taken in account how she wanted to use Corsola with Psyduck sabotating her plans. That same Corsola which defeated Trinity Gyarados(trainer who ended as runner up in that tournament being fairly famous for her skills as trainer).
Likewise there is no evidence to support how Totodile is Ash "weakest" pokemon. Especially when it never had chance to be used after Johto to show his true potential like Ash other older pokemon like Heracross, Noctowl, Quilava, Torkoal etc. All received chance to be featured in newer regions.
Most of all Totodile wins over Kingdra, well trained Charizard in battle park or Harrison Sneasel dont give picture of being weakling.
p.s. It also passed around 10 years last time Misty was seen(last in person return happened in July 7th 2005. in main series), so we cant act like everyone else became better and she stagnated not improving, getting more powerful pokemon, developing more productive strategies and possibly collecting experience on solo journeys(if we take in account how she has higher dreams than to be gym leader) either.