The common wisdom is that flexier frames are better for spin [all other things being equal].
The reasons for this could probably get technical, but basically, the very slight 'give' of the racquet allows more dwell time, which theoretically allows for more drag across the ball, meaning more spin.
I think there's another good reason, which is that the ratio of power : spin is lower (IE the smaller amount of natural power makes it look like the ball is spinnier) because flexier racquets are almost undeniably less-powerful [all other things being equal].