Is it safe to delete the library of migrated photos?

Yes, if you are sure everything you need is in the photos app library.

However, there are some file formats that have been "depracated" by later version of MacOS, and if you have any of these in your iPhtoto libray they won't have transferred. It is a good idea to archive a copy of the migrated library to an external drive, in case you ever find something missing.