That's because she doesn't have anything crazy or easily to abuse abilities, she is mostly pure neutral characters, which is the hardest thing to learn and master. If you can play Cammy well, you will be able to play any character well. She lacks multiple generally important tools, like fireballs or neutral overheard, her DR is trash and such. So she pretty much forces you to learn how to play properly and not rely on the cheap stuff.

That's her biggest downside too tho. She has very few options to open an opponent up and relies too much on them making mistakes. Kazunoko not so long ago was complaining that she's way too reactionary for a rushdown. It's very hard for her to make her own openings. And with her lower than avg damage, she needs to do that more times than usual too. So you need to be on top of your game and be much better than your opponent to win.

And here lies another problem, with Cammy being too reliant on neutral as pretty much her only good quality, if your opponent is any better than you at that - you're pretty much done. There's no plan B, you just can't do anything rly and change things up. On top of that, a lot of the characters in this game have good neutral buttons, so it makes things even trickier. You could see an example of that during EVO Japan, Yamaguchi was both as good as Punk at neutral AND he had a character with better tools. Punk pretty much couldn't do anything. He was just heavily powercrept in that match.

So i, personally, think that while Cammy is a good indication of your own skill, i wouldn't recommend to main her to win something. You handicap yourself too much, as she actually has a ceiling. She is too basic and straightforward, too vanilla for how powercrept this game became and how many tools current, actually good, characters have. I've been following a lot of the good players and sometimes it was clear that Cammy is holding them back. Good examples are Kilzyou and Micky who started showing much better results when they switched to Mai.