'So so'
I think in textbooks and classrooms the word 'so so' is taught in every language (for example in French - 'comme ci comme ca') but i dont think it's very common. At least not in english.
Maybe this will help - i'll write a list of terms going from 'very very good' to 'really bad'?
'Awesome!' - really really good
'Fantastic' - really good
'Alright' or 'Pretty good' - good ('alright' actually means less than good but you say it to be modest. So usually people know you actually mean 'good' even though you're being modest by using a word than isnt so good)
'Not bad' - less than good... it's okay, but not so great. (i think if you translate 'not bad' into chinese you get bucuo but in chinese bucuo is actually quite good but in english 'not bad' isn't so good)
'Pretty terrible' - bad (the word 'terrible' actually means really really bad, but when you put 'pretty' in front it makes it less bad)
'Awful' - really bad
'Terrible' - really really bad