There is no point in Appearing for IIT JAM for admission to M.Sc. Courses in IISc/IITs/NITs unless you are in the Final Year/Completed B.Sc./B.Sc(Hons) in Physics.

Hence you need to show degree/ enrollment in an B.Sc. Course, irrespective of the Hons status, if you appear for IIT JAM followed by admission to Respective Higher Institutes.

And definitely a B.Sc. Physics student will have a better conceptual understanding of the Subject, hence better result in IIT JAM Physics.

For more information about IIT JAM Physics do follow the link below

https://university.careers360.com/articles/how-prepare-jam-physics/amp