Not at all. Unlike official Dolphin (which updates every few days), MMJ is updated only when the developer implements a major performance patch. An older verified version is safer than a "new" version from a sketchy site.
Try the official Dolphin first. If performance disappoints, grab MMJ from GitHub. Don’t forget to verify the SHA-256 checksum if you’re extra cautious.
: Enhanced utilization of multi-core mobile processors. How to Find a Verified Dolphin MMJ GitHub Download