I feel like a recurrent problem with many of the girls, especially Misty, May, Serena and the SM lot, is their tendency to struggle having an agency and reliance on plot armour or 'falling upwards' unless the plot is tailor made for them specifically, at least as a trainer. Yeah, there's a lot of cases they push themselves and are determined in a tough spot, but in terms of them actually figuring out an impressive way to win, it's not as common as the plot just saying 'oh well, the gesture's enough' and just plot armouring it for them. The boys suffer from this often as well, but I feel like it's the girls that tend to get exceptions the least, maybe because they always get the 'table scraps' after Ash in terms of developed battles and action where they're clearly using their brain (also I feel like they're more liable to subvert this sometimes and just let the boys lose miserably unless they do well too, meaning they don't skip over an accomplishment).
I think this is why so many like Dawn, and are starting to have a hindsight respect for Iris, since they could force themselves into the plot by sheer force of personality, and tending to get equal treatment in terms of trainer development. Even Lillie I think was more liable to get some involvement in random episodes (even if I think Sophocles kinda supplanted her as Ash's intellectual assistance in the end).
Looking back at Misty and May I think the recurrent problem with both is how often they rely on dumb luck or random boosts they had no real control over. Misty kept winning just by Psyduck's DEM headache powers and never really learned to control him properly onscreen or got many battles he didn't butt in and she got to show she could work on her own. She started off with an okay agency in Kanto as an actual character and foil, but as the formula started to fasten they just stopped bothering with her outside her running gags.
May had an effort to give her an arc but as mentioned her contests tended to not very developed well, she relied on spamming or plot armour a lot of the time over well choreographed battles. Also AG wasn't terrific with supporting roles either, she had an okay bond with Ash at times but otherwise tended to sink into the background unless it was a focus episode for her. The biggest exception seemed to be her high tendency to get kidnapped or put in peril. Even the Manaphy movie, which felt her limelight movie, ended up with Ash saving her and May failing to have much actual agency.