Passable, middling (4)
(Other definitions for soso that I've seen before include "mediocre" , "Indifferent" .)
I believe this is a double definition.
'passable' is the first definition.
(thesaurus)
'middling' is the second definition.
(thesaurus)
(Other definitions for so-so that I've seen before include "Of middling quality" , "Of average quality" , "OK, but nothing special" , "Indifferently good" , "Mediocre" .)