Can I use a computer UPS with an oscilloscope?
1- does computer UPS give isolation from main supply? ( like alternative to isolation transformer)


No, "UPS 600VA" it is not isolating from main supply


  2- output of the computer UPS waveform is "simulated sine wave", does if affect performance of oscilloscope


No it doesn't affect the performance because it converts to DC inside the oscilloscope.

A simulated sinewave is basically a pulse width modulated squarewave, with a proper output and/or input filter i would not expect any issues.


  shall i use computer UPS for oscilloscope


No, I don't see any reason for this unless you live in a place with a lot of short-term power outages.