It does look like the 240p test component images contain compression artifacts. However they are also sized closer to 512×480 (508×472) instead of the 720×480 output of the RetroTink so they have been downscaled.
They are also very dark. Maybe you have the brightness switch on the HDRetrovision cable set to the wrong position?
Have you not been able to capture images directly without the DVDO? If the issue is the refresh rate of the SNES you can set the 240p test to 480i which will make the SNES output at 59.94Hz. Although you should probably get a better capture card like the Datapath VisionRGB E1 which will accept anything you throw at it and allows for 4:4:4 capture.
EDIT: I found out why the 240p component images have artifacts. They are 256 color png instead of 24 bit.