Why have pictures on Instagram got blurry since my iPad has been updated to iOS9?
My suspicion is that the Instagram app was programmed for the older OS and needs an update. You could check their support forums, but it also might be something you just wait out for the next few app updates to post from the vendor.

It also could just be server side changes - if your iPad isn't served the complete resolution images, you won't have them. (i.e. the problem might not exist on your device at all).