Aseprite exports very sharp files in my experience, are you sure it’s not the viewer you’re using that’s making it appear blurry? For example, social media sites will scale your art up, blurring it. If you need to post your art somewhere where it’ll be scaled up, export it scaled up by some amount (Aseprite offers a pixel-perfect scaling option in the export dialog), so that when it’s scaled, it won’t be as blurred.