Can the UPS give isolation from main supply? (As an alternative to an isolation transformer.)
Only if the UPS provides isolation (and it would have an isolation barrier internally) Some UPS's convert AC to DC and then back to AC with switchers.
The output of the computer UPS waveform is "simulated sine wave." Will it affect the performance of the oscilloscope?
It really depends on the setup, but if the ground of the oscilloscope is isolated from earth, then it's most likely floating. Whatever your measuring would need to be plugged into the UPS or there really wouldn't be a reference for the oscilloscope ground.
Whatever your measuring also needs to be grounded to the oscilloscope.
This is also not a good way to measure any high voltages because the ground of the oscilloscope would need to be connected to the high voltage device and that could create a saftey hazard.
The best way to make isolated measurements is with a differential probe because it has two terminals and determines the voltage between them