AEM Mocks Core is not buildable in maven because org.apache.commons:commons-imaging:jar:1.0-R1534292 can not be found in maven central repository.
The problem can be fixed, by using the existing versions 1.0-alpha2 or 1.0-alpha1.
the missing dependency can be found at
also adobe is in progress if migrating all it's artifacts to maven central some are still only found in the adobe maven repos
switching to 1.0-alpha1/2 is not an option as we need to stay in sync with the version included by adobe in AEM, which is unfortunately only a snapshot that was never released - the referenced version 1.0-R1534292 comes as close as possible.