Phantom 3 standard user here. Was just normally flying, and this happened: https://www.youtube.com/watch?v=OnUts3jKEkQ&feature=youtu.be
is it bad?

Was that movement often or this time only? If it´was this time only then it´s not so bad,if it´s permanent then you may take care for the yaw axis of the gimbal.
What FW is on your P3 system?

It happened like 2 times out of like 15 flights. Also don't know what a "FW" is, sorry.

If you want to know what FW version on your system then power up the RC and AC as the mobile and start the Go app. When the cam feed comes then the status menu opens and show you the version of  the FW as latest and the current status of the RC/AC system. The version number is available over the settings menu upper right tab the 3 points and then AC.

The app version is 3.1.52 and the "version" is 1.9.20

Did you have a look into the flight logs yet? There should have an entry about the cam movement inside.

No, I didn't, how should I view them?

Look into the drawer "djipilot/log". The flight log is in txt or csv.