Online proofs do not indicate the actual quality of the printed work. They are meant to depict the trim edges and to make sure that nothing is missing or getting cut out from the artwork. To make the proof easier to download to your computer they are made low resolution. If there are any issues with the resolution of the actual work there will be a warning on your proof about it. In that case, you can reject and resubmit the proofs with a higher resolution if you choose.