Don’t use JPG/JPEG. Those are the same format and they will always degrade the quality of pixel art. PNG should be fine.
Where are you viewing the image that makes it blurry? My guess is the image is actually fine, but you’re viewing it enlarged. Most image viewers are not designed for pixel art, and will blur images when they scale them up, and this is much more noticeable on small, sharp images like pixel art tends to be. If you want to make sure your art remains sharp even when viewed large (e.g. on websites, in social media apps, or in most image viewers), you need to pre-upscale it yourself using Nearest Neighbor interpolation. Aseprite offers a scaling option in the Export dialog for this purpose.
In the future, please search the forum before starting a new thread. There have been a LOT of threads about this issue, and it’s pretty much always the same answer xP