The way I read it, you're wondering if an isolation transformer is needed to prevent the spikes and dips common in an environment containing inductive loads (e.g. motors) from influencing the input power on a particularly sensitive piece of measuring equipment. My semi-educated answer is, if the UPS is online, i.e. ALWAYS provides power by converting DC from its constantly-charging batteries to AC, then power line fluctuations should not appear on its output, as its batteries will filter any variations, so further isolation should not be needed.
I fully agree with jtt283. If you're using a good-quality UPS that is ALWAYS providing output power and recharging itself, that unit should be all the protection you need from power supply noise and spikes. However, low-cost UPS units are available that only keep themselves charged and do NOT supply any output when line power is available. In "normal" circumstances they just allow the power supply lines to feed directly through to the load, with some filtering to remove power line spikes. The filtering may or may not be very good. These units attempt to detect a power supply failure very quickly and switch the load from power line direct feed to UPS output feed so that the load is kept running smoothly.
Read the specifications of your UPS very carefully to determine how good it is in preventing power line noise, spikes and sags from reaching the load, and whether it is always on. A good unit, especially one that does deliver clean sine way output, probably does all of this well.
im not an expert but from my understanding an isolation transformer between the ups and wall may keep ur expensive UPS around longer. Get one rated slightly higher than the UPS. I know one of the worst loads for frying expensive computer equipment is high power welders. large motors will create dips and spikes as well.
isolation TF alone wont fix everything but a decent sized Online UPS is going to run you north of $700 putting a $200 isolation transformer in front of it may make it so you arnt replacing an expensive ups bi monthly if the spikes are bad.
yes the battery filters the power, but remember there is an AC to DC circuit in the UPS which must charge the battery, the companies wouldnt make much money if you could just replace the charge and protection circuit. you will have a slight trade off in efficiency but if the gear is expensive/mission critical behind these devices it is well worth a 15-20% efficiency hit for the total power consumed.