Great question!
Well, all music is musical pretty much by definition, eh? Or not.
Musical is a totally subjective term. Practically, it means people like what they hear.
Distortion is the key.
Some people like no or low distortion and hear that as more "musical". Low distortion is a proper objective goal, but "musical" is a subjective term and not the same.
Others may find the presence of certain types of harmonic distortion that does not bother the ears more "musical" because it sounds better or right to them. That’s fine....it’s a totally subjective determination.
If you read up on tube amps you will find that they tend to have unique distortion characteristics that many find more "musical", especially with how the human voice and acoustic (as opposed to electronic) instruments sound. Others not so much
On the flip side, electronic amplification and sources are an integral part of a lot of pop(ular) music. The unique distortion characteristics of tube amplification tends to fare less well there, though there are no absolutes. Both tube and SS amps are capable of producing similar very high quality sound for a cost.
Tube versus SS sound is just one common example where the term "musical" often enters the conversation. All it tells you is what each person prefers ie what their own personal opinion is of what sounds good or not.
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