Not bright or colorful.
Example: "The lighting was too dim for me to make out his facial features."
Not smart or intelligent.
Example: "He may be a bit dim, but he's not stupid."
Indistinct, hazy or unclear.
Example: "His vision grew dimmer as he aged."
Disapproving, unfavorable: rarely used outside the phrase take a dim view of.
Lessened, reduced
Example: "The diminished roman empire never recovered from the sack of rome."
Made to seem less important, impressive, or valuable
Example: "She felt diminished by the report card."
Reduced by a semitone