You can check any external requests in a source code if you're not sure. This is the benefit of Open Source.
If you are not a programmer — ask your friends, who you trust.

This script is safe for my opinion.
All that it's do — add colorful badges for every cattribute and also calculate the average price for kitties in a marketplace (based on last trades).
It uses only built-in calls to CryptoKitties API, so I don't see anything criminal here.