Mud is actually something that applies to the whole frequency range. You can have mud in the center, on the sides. Great gain staging compensates for mud - at the cost of the dynamics. For instance a loud sweet mellow sounding electric guitar panned widely to one side should have a low pass filter, yes low pass filter.
I know many mixes that appear rather clean, but when you pay close attention to them various frequency ranges are contracted, it is like the mix is "locked up". This and what it does I find to be a form of mud as well. When the waveforms of the various tracks combine various frequency ranges can have more cancellation than others at various sections of the mix. When you have a mix that has cancellation at the "wrong" frequencies at the wrong places it can cause the perception of mud. Every recording is unique so there is no way of predicting this, but it is essential to go through phase optimization properly. If your mix tends to easily get low end intensive, it might be that you need to change the phase relationships to effectively get rid of it. I like to optimize phase in mastering, because I can hear it the best when the content has been maximized the most.
Another thing that is very common among mix engineers, I've seen this even among people writing books about mixing, is to achieve balance at a low overall volume of the mix, say -16 LUFS, to leave a lot of headroom for mastering. That might "appear" mud free at that level, but you are most likely going to find out it's very muddy as soon as you bring it up to a commercial level in mastering, so now the mastering engineer has to deal with all the mud, and he/she might not. In my experience balance is loudness fixed, that's why balancing into compression works so well - when your mix is in balance and it is so close to the final loudness level, then the mastering engineer can bring it to the final level without having to re-work the gain structure of the mix. I think it is optimal to mix into compression slightly below the final loudness level. That forces you to fix balancing issues earlier in the overall process and when you hand over the mix for mastering it is more likely the mastering engineer will find it ready for mastering.