If it would be possible to share a link to an example file where I can reproduce the issue myself, I can share it with our developers. You could create a duplicate of an existing file and set it to be accessible to anyone with the link, and then share it here.
This is a frequent complaint from users of Adobe Express for their web/social media images. Bottom line, Adobe Express outputs to a lower file size to save bandwidth. Unfortunately, saving bandwidth can reduce image quality on certain high-pixel density displays like Retinas. However, average density display users don't notice much difference in quality.
For my own web projects, I'm using Photoshop CC and SavingAs WEBP file type. WEBP generates higher picture quality at 25% less bandwdith than a comparable PNG file. You can read more about WEBP files below.
That said, I don't know if Adobe Express or Etsy support WEBP images yet. Although they should as most modern browsers do support the improved file format.