MantisBT is free software. We encourage you to dive into the source and help on new features and bugfixes. That is the way free software works.
Here are some important pointers for MantisBT-Development:
MantisBT sourcecode is hosted in the GIT SCM. Use gitweb to browse the source. Detailed information on how to use git with MantisBT can be found in the Mantis Bug Tracker Developers Guide
GitStats for the MantisBT GIT-Repository are available at Mantisforge.org.