Misty wasn't even at the Gym when we first saw her. She was fishing near Pallet town, she probably ran away from the Gym as she was tired of her sisters and didn't battle challengers there. Nothing said in those early episodes made it seem like Misty battled challengers regularly. I think Ash was the first one she ever battled in a Gyn match.
You claim Misty is a better trainer, but there are no examples. None of Misty's early Kanto battles come off as impressive, her Starmie
loses to a Graveler
in the Poketech ep, and then she hardly battles any other trainers besides TR.
While Misty did improve at battling in Johto, she certainly didn't come across as strong in Kanto. Ash nearly beat her in the Gym as Starmie
was on the verge of losing energy before TR came.
As for Excadrill
does it not seem more powerful than either Staryu
or Starmie early on? Starmie was losing to Pidgeotto
who Ash just captured like 4 episodes prior.