"Mal so, mal so."
This one puzzles me. "So so" means "not so great, not so bad" in English, whereas I think "mal so, mal so" (sometimes one way, sometimes another) in German could mean the same thing in certain contexts (Das Essen in dem Cafe ist mal so, mal so.) But I would translate that as "inconsistent," rather than "so so."
Two other translations I found were: "To blow hot and cold." and "first one thing, then another"--both of these convey the core (German) meaning of inconsistency or inconstancy, rather than the English meaning of mediocrity. If you say "The coffee at work is so so.", that means it's mediocre, or "blah," not that it's sometimes good and sometimes bad.
Yes, I agree. To collquially express mediocrity in German you can use 'geht so'? 'Wie ist der Kaffee in dem Restaurant? Geht so.' - 'How is the coffee at the restaurant? So so.'
I thought the German word for "so so" was "solala" (don't ask me how it is written).
Actually, I would say that "so lala" is a better translation for the English "so so", because it implies mediocrity, whereas "mal so, mal so" implies inconsistency.