Lo-Fi tends to reward a specific EQ shape — here's a starting point built for it.
Try +3dB at 60Hz, +1dB at 500Hz, -2dB at 2.5kHz, and -3dB at 8kHz. Rolling off the top end and adding warmth in the bass recreates the mellow, muted character lo-fi listeners look for.
Lo-fi as a style intentionally imitates the limited frequency range of old cassette tapes and worn vinyl, so cutting both the high-mid and high bands here is recreating a specific sonic character on purpose, not fixing a flaw.