Top 3 Reasons Why Ran Mouri Is A Useless Character:
1) She doesn’t know about the BO, so she can’t be heavily involved with Black Organization plot.
2) Nobody tells her and keeps secrets from her to keep her ‘safe’ .
3) NOBODY TELLS HER ANYTHING. THEY COME UP WITH INTRICATE EXCUSES OR STRAIGHT UP AVOID HER QUESTION. (See the Eri Kidnapping Case and Betrayal case where Conan had an alibi ready when Ran confronted him with ‘Are you Shinichi?’ line of questioning again and Vermouth disguised as Azusa and telling Ran to ‘not come into her world’ when she asked Vermouth is she was the one who called her’Angel.’
So, maybe try to see the plot from the character’s pov rather than bashing her for things she doesn’t know about because people around her don’t want her to know.
Ran’s smart and she has suspected Conan so many times and has confronted him too but every time logic wins out because how can someone de-age? So, she takes the logical route and drops her suspicion.
Try to see the character from her pov rather than putting others’ on pedestal.