I'd be rather shocked if such a film had a poor soundtrack.
For starters it was all in the 60s and early 70s so there was a whole raft of great music to choose from. Then all of the films were made retrospectively some years later and so there was some indication of which songs would stand the test of time.
But I think that is true of any era or any year. There is always great music around what diminishes is our willingness to seek it out so most people rely on the same old same old. The thing is you don't realise that it is happening. For me this started to happen around the early 2000s. I still went to the odd gig and listened to radio and watched music programmes and so on. I thought I was still "involved" but I wasn't. Not like I was 20 years earlier when I didn't even have a TV and spent loads of time at gigs, parties, festivals and so on.
I think this happens to all of us to a greater or lesser extent but accelerates hugely when parenthood arrives.
As an example look back at the Rock 'n' Roll Years series from years ago where they soundtracked news events from a year with hits from the time. Every year was interesting because you have a whole year of global history to choose from and if you can't find 30 or 60 minutes of cracking songs from any year you need to sack your researchers. Note that this is all retrospective again.
Compare and contrast to a contemporary movie. There's all sorts of 60s films with dodgy soundtracks and equally there's many with banging ones but that is true in any era too.
OK so The Cheeky Girls have probably made very few soundtracks but The Spice Girls had a movie to themselves and it is awful. Just as most films made to cash in on an act's current success are. Elvis made lots of films, for instance, but mostly they are pants.
So I reckon if you set a film in any past era and give it 10 years then you are almost nailed on to have a killer soundtrack but set a film today and choose today's hits only and you are far more likely to end up being embarassed by the passage of time.