be not so much something as something

be not so much (something) as (something else)

To be more accurately regarded as (something else) than (something). The phrase is used to emphasize that the thing being discussed is more aptly described as the adjective after "as" than the one before "as," even if the first descriptor might also accurate. I'm so sick of this project. It's not so much difficult as it is boring.
See also: much, not
Farlex Dictionary of Idioms.

be not so much something as something

be one thing but also something else which is more important: He’s not so much unintelligent as uninterested in schoolwork.
See also: much, not, something
Farlex Partner Idioms Dictionary
See also:
  • appropriate for
  • than a bygod
  • bulletproof
  • man to man
  • man-to-man
  • muck-raking
  • a few
  • pearl-clutching
  • as good, well, etc. as the next person