Jim Culver is an investigator whose power level varies wildly depending on which difficulty settings you play with, and that's a big reason why Jim tends to vary so much in tier lists.
For most play groups on Easy or Standard difficulty, Jim is a Mystic investigator with a mediocre stat line and strong synergy with card interactions that involve skulls. Outside of Jim's 4 Will, none of his stats are particularly noteworthy, his signature Jim's Trumpet is a mediocre source of sanity healing, and Jim's primary appeal will probably be 5 level 0 cards outside his class. The ability to treat Skulls as "0"s isn't that attractive when the worst modifiers in the token bag usually cap at "-3" or "-4".
For play groups that play frequently at Hard/Expert difficulty, Jim becomes a functional god of the token bag. Skull tokens are relatively mild in most Standard/Easy Scenarios, but Skulls scale upwards at high difficulties until they're functionally auto-fail tokens, so Jim becomes one of the few investigators who can consistently pass skill test by fishing "0" tokens or skulls out of the bag (with cards like Dark Prophecy, Nksoi Mabati, or Grotesque Statue. When the token bag is full of modifiers like -5 or -8 and you have to overcommit by 2-3 cards to guarantee a success, Jim becomes a godsend. Jim doesn't need a great stat line or level 1-2 cards from other classes if he can near-guarantee drawing a "0" modifier on important skill tests.