Just meant that it's missing a distinctive low end layer that would give more contrast to the track, but it might changed the intention for the track as well, so it depends what your after for this one, but I think that something like a sub bass or even bass synth or pad layer could add something. As it is, it's the "clicky" attack of the kick that is taking over, try another kick and see where it leads without adding another bass layer, should change a lot already.
Well, you can't really pin point frequencies for this, Mud is more about having less transients and clarity or when sounds feels like pushed back and not being in the front ( think about depth, stage, distance ), it can happen with compression and too much reverb decay time, or overlapping/dueling sounds in certain frequencies. It's not the case in this track, so no issues with this here.
All of that said, terms like "Mud" are quite subtle, it's usually used to say that there's conflicting sounds in the Mids, Low-Mids frequencies, but basically it's when a track lacks too much clarity.
If cutting less using Hi-Pass filter doesn't give you good results or leads to sounds fighting for space and loosing clarity, then try to use saturation, you'll keep your fundamental low end frequencies but saturation will add harmonics in a higher frequency range and that will help to push the sound forward in the mix.
With sounds processed with effects like echos or delay, timing, decay and feedback settings will give very different results, so it can help to adjust this as well.