It is technically correct, but unnatural. The more common expressions would be "I am not as tall as you" or "I am shorter than you."

Hi Maggie, I agree with Nathan's response. If I were to say, "I'm not as tall as you" or "it's less expensive," I'm probably trying to stress the comparative adjective "less." For example, "something is less expensive than I originally thought" or "she's not as tall as me," meaning that I really want to draw your attention to the nouns "expensive" and "tall" so that I can really highlight this negative comparison. If not, I'd just say, "it's cheaper" or she's shorter." It's more common. I hope this helps. Best, Wenwei