Do you just need DC isolation, or do you also need low coupling capacitance because you want to float the oscilloscope to probe floating nodes with a high dV/dt (like high-side gate drive in a SMPS)?
Any CM noise from the DC/DC converter can be addressed with filtering, but the filtering and coupling capacitance also impacts the rejection of external noise, which may be a bigger issue than the noise generated by the internal DC/DC converter.
Common noise is going to couple into the oscilloscope measurements through voltage drop on the oscilloscope leads. The ground and signal connection have different impedances which leads to common mode to differential mode conversion.
Any common mode noise generated by the isolated DC/DC can be shunted with a sufficient capacitor across the isolation barrier and adding common mode chokes on the input and output. However, the larger this capacitor gets the lower the impedance for CM noise between the USB and the oscilloscope circuit connections (noise can be induced in either direction). USB (or what you are probing) can be quite noisy, so its possible that these sources will be a bigger noise contribution than the internal DC/DC converter.
I was working on a USB powered rogowski coil recently and used one of those cheap little 2W isolated DC/DC modules which seem to have ~20pF of coupling capacitance. With a cheap USB power supply separate from the scope a few mV of spikes were induced on the input to the oscilloscope due to common mode noise from the USB adapter. When the probe was powered from the scope there was very little noise as there was no CM noise between the USB power input and BNC output.
https://www.digikey.com/en/products/detail/mornsun-america-llc/A0509S-2WR3/16348392Just following the manufacture recommendations for the filtering of the DC/DC and a small SMD common mode choke was enough to push the noise from the DC/DC itself down to the noise floor.
I got adequate performance for my application with the cheap DC/DC module. If you want lower noise the bigger issue might be rejection of external CM noise rather than CM noise generated by the DC/DC itself. A low coupling capacitance would help with that.
It also might somewhat become a matter of how big a common mode choke you are willing to use.