Currently, ImagePerfect only officially supports reading tags and ratings written by Shotwell, and only if you had Shotwell write that data into the image files themselves (via its “Write Metadata to Files” option).
If that’s your setup, make sure to scan the folder for metadata before adding or changing ratings/tags in ImagePerfect.
ImagePerfect is non-destructive, with the only exception being that it writes ratings and tags directly into your image files. Scanning first ensures your existing Shotwell data is loaded correctly and helps avoid accidentally overwriting it.