I really felt that May never really grew much as a character. She stumbled into contests and almost instantly mastered them pretty early on, because of course she couldn't go on disliking Pokemon for the entire season. Dawn, by comparison, really struggled: she went through loss after loss, had to get her priorities straight, and there was a clear progression in her character and in her strategies, as she spent so much time training - on-screen - with Ash. However May was rarely, if ever, shown training and practicing for a competent win, instead relying on luck, "hax", and borderline deus ex machina for her wins. May is the kind of character who opens the episode with "My Pokemon and I have been training extra hard!".
May was never really more than a side character with a side character's amount of screen-time and character development. If she needed to train and learn a new strategy, she did it off-screen. Hell, her contests were sometimes off-screen as Ash went off to train and the show focused on him. Where as characters that came after her by comparison, practically upstaged Ash and took control of the anime for themselves, more than once. I was really impressed by the amount of thought and attention given to them, and found myself empathising with them so much more.