@egalandbooks there is nothing particular about mastodon that would make them blury. It may be the resolution of the originally uploaded image, or the instance the user is on may downsize them to save space, or your instance may do the same. (I have heard many admins complain by the far the biggest user of space is images from posts from users that their users follow).
If you directly access my Phot Gallery it will look fine on your browser, I don't think Mastodon lets you access a whole gallery, you'd only be able to compare posts that show on your instance that contain such images.