Shampoo doesn't "improve" because she was purposely crippled by Rumiko in retaliation to her popularity.
Translation: Rumiko didn't want what happened with Lum to be repeated, and have her readers root for the crazy and fairly murderous girl because she's "exotic" and had big tits.
She's already vastly superior to all the other girls in terms of physical ability and when she first shows up Ranma immediately flees on sight rather than try to fight her.
...Because she was fighting to kill, and Ranma wasn't going to reciprocate. He ran and hid because it's the practical thing to do when dealing with an assassin that you won't kill or maim due to morals. He beat her in one kick when she first challenged him, and he took her out in one move when she threatened Akane. I really don't think that Shampoo was ever intended to be portrayed as someone close in skill to Ranma. Superior to the other girls, sure, but her feats in her first arc
pale in comparison to the stuff Ryoga pulled off when fighting Ranma.