Yes, this mismatch is there, but how important it is depends strongly on your requirements.
You will have power losses with a big contribution that depends on the frequency:electrical length ratio.
There are frequencies where it will be almost losless and frequencies where the loss is more visible.
If the electrical cable length (about 0.66 times physical length) is 1/2 of the vacuum wavelength of the signal, this mismatch loss will almost disappear and mostly connector and cable losses remain.
Also, the interface of 50 Ohm to 75 Ohm will be visible in TDR applications.
Anyway, use it and replace it when you are in doubt that it is suitable for your needs. I consider it OK for SDR (broadcast is 75 ohm standardized anyway!) and everything else - try it, and if something that you use the cable for looks fishy, consider its the cables fault.