"Terrible" is rather vague although one can imagine what might be at issue.
Room acoustics, especially reverb, is nearly impossible to remove. If it's heavy in a specific frequency range one can EQ/Notch that area out. One can use a gate to get rid of reverb tail. Once can use Noise reduction to get rid of some aspects of the room noise. Make noise print from an area with room tone only.
It's hard to be any more specific than that. Depending on the circumstance "better" may or may not be possible. This is why one avoids camera mics to begin with. Depending on the mic problem, it's often better to deal with that issue on site and immediately even if it means holding up the shoot a bit rather than spend hours in post making very marginal improvements with bad audio.
When a producer decides to use the camera mic they must entertain that decision knowing full well what they may face.