cannot import pics from SD card to iPad with USB-C reader, but lightning reader will import same pics to iPhone

DId you Open the photos app and none of the photos on the SD card showed?

1. Open Photos on your iPad, then tap Import.
2. Select the photos and videos you want to import, then select your import destination.

• • Import all items: Tap Import All.
  • Import just some items: Tap the items you want to import (a checkmark appears for each), tap Import, then tap Import Selected.

With introduction of iPadOS the import photos prompt doesn’t now occur. Instead, you should be able to simply import using the native Files App.

More info here (link updated):

Import photos and videos to iPad - Apple Support

Otherwise, simply use the native Files App to copy files from the connected USB Card Reader to you iPad. When connected, the Card Reader or USB Storage device will be visible in the Files App sidebar. If not visible when connected, tap Edit at top-right of the sidebar, then add the connected device to the sidebar view.