Interesting, but if I may play devil's advocate: that was Kanto, when Ash was relatively inexperienced. Since then, he has conquered the Battle Frontier, come oh-so-close in multiple major tournaments, has trained multiple teams across multiple regions, and has managed to succeed with the frail Electric--which lacks its HA by the way, has a crappy movepool in general, and has spent three generations running redundant STABs (not that that matters)--as his ace. Basically, Ash is equivalent to one of those Ace Trainers you'd run into on Victory Road.Sorry to bring up a dead topic, but I do want to point out that Misty does have one contributing factor to her popularity in the west besides First Girl.
Misty is the only one never portrayed as inferior to Ash when it came to battling. While this doesn't sound like much (Ash was literally just starting and she's a gym leader), it set the tone for Misty's character as a mentor and later equal battling partner (when Ash got more experienced. Sorta). Misty was never the protege, she was always a competent and would fight alongside Ash in non-official matches (i.e. Team Rocket) and even compete against him sometimes (Togepi tussle, Whirlpool Islands Arc, the Latias/Latios movie race for example).
Or, at least she started that way. As the Kanto and Johto arcs progressed, it seemed the writers tried to nerf her with Psyduck, giving up Starmie for a while, Togepi (a lot of people blame Togepi for "ruining" Misty's character). Plus she was a little gimped from the get-go, since Goldeen and Horsea were nigh-unusable outside of water or ocean episodes. So thank goodness for Staryu? At least they started to back off of this once Poliwag and Corsola were caught, even eventually giving her a Gyarados.
While "strong fighter" does not equate to good character, it does leave an audience impression. And it's not to say that the other girls never became equals to Ash (I particularly liked May's farewell fight), but they were always catching up with Ash. Misty is that sole exception.
You can argue they tried to pull another Misty with Iris, but giving her by all accounts a baby Axew and an unloyal Excadrill to start with but her squarely in the Ash's protege file. Even in the currently season, the role of the Misty and Brock characters are merged into Clemont. I don't think Serena has ever fought against Ash yet, and I've rarely heard of her assisting in defeating Team Rocket at least (but I might just be not paying as much attention).
TLDR: Misty popularity is inflated due to being the first girl, but she's not JUST the first girl.
Here's the amusing part. His female companions after Misty are, in order: player character/rival (May), player character/professor's assistant (Dawn), eighth Gym* leader and future Champion (Iris), player character/rival (Serena). I can see what you're saying--this ought to be a competent bunch in any right, although Serena's arc is distinctly different from the games, so I'll let that go.
*--Note that the anime uses a modified order of challenge--Roxie, the second Gym in BW2, is the eighth in the anime.