Mixing cabs can be totally fine. The keys are to either "get lucky" with a combo that just happens to sound good together, or use science to predict a good match.
"Mix bad" only applies when both luck and science are against you.
Pretty much anything you do other than mixing two cabinets that aren't designed to work together will be a more productive use of your time.
Spend it learning how EQ, effects work. Spend it practicing. Spend it working on your bass setup or trying out new strings. Spend it doing wrist and hand exercises.
Hell, mowing the lawn is about as likely to have a net benefit on your sound as mixing random cabinets.
If time was not limited I'd say go for it, but really, it's so likely to be counterproductive that you shouldn't do it until everything else is perfect. In my opinion.
) that '70s vibe is just a turn away.