Pop music has always been stereotyped, or at least standardised in some way. Songs of the 50's invariably had the same form: V1, V2, bridge, V3, repeat bridge, repeat V3. Songs of the rock & roll era almost invariably followed a 12-bar blues chord sequence. To look at a different aspect, styles tended to alternate between simple "D-I-Y" (think skiffle) and more developed (think Sgt. Pepper), the simple styles showing less variation. Dunno about the present day, I gave up on pop as a bad job thirty years ago.   

It would, however, be interesting to see how pop compares to other music genres under a similar analysis.