Hyde here from Shopify.
Instagram feeds are usually implemented through a third-party app or theme. While it's imperative that you have the best quality images uploaded onto Instagram, the feed itself could also be reducing the quality of the image.
So it's worth having a chat with the developers of the theme or app to see if there are any image specifications or requirements you need to meet, or if they have any suggestions on how to improve the quality.
I hope that helps Svetlana, happy to answer any other questions you may have!
All the best, Hyde.