the sars-CoV2 virus RNA is unstable in the region coding the glycoprotein of the ACE2 receptor making it more susceptible to rapid mutation in that region of the RNA...
Unstable in some strange sense or prone to mutations?
That molecule being unstable thermodynamically (even if it is, which I doubt) makes no difference.
It is a single strand RNA, acting also as an mRNA, so that fact that it is "unstable' does not preclude it from functioning in directing protein synthesis..
Typically, mutations occur during viral replication.
Natural selection favors variants which are more infectious, as simple as that.
Right now it does not matter (for viral evolution) how it came out to be-that's a different story.