Many sucessor states seem to effectively award their mechwarriors the rank of sergant in the military. the DCMS distinctly does, and the AFFS awards anyone who has graduated the academy the rank of sergent. I suspect this is the case for the Lyrans too, and wouldn't be suprised it it was near universal
I think a lot of it depends on how top-heavy or officer-heavy your military command chains happen to be. Someone with more recent Army experience than me mentioned that the bottom rank for chopper pilots was Warrant officer, and that's been true for a few decades, but at 51, I remember when the Army got rid their last non-officer/non-Warrant officer pilot ratings-so it wasn't as terribly long ago as you might think, and IS linked to some of the rank inflation we saw in the post-nineteen-seventies military (particularly the post-drawdowns of the early 1990s where there were a LOT more officers with a lot fewer slots after the festival of RIF post Gulf War and pre 9/11.)
That said, 'mechwarrior isn't something you can just fill. At least, if they're still keeping consistency with the older editions, roughly 1 out of 1000 people have the neural conductivity to use a Neurohelmet, out of those, only a fractoin (depending on harshness of training) will be able to learn to use it, and only a fraction of those, are going to be able to complete the process of learning how to fight as part of an army.
and only a fraction of the ones who can fight, can also lead.
So it makes perfect sense that a fresh-out-of-training 'mechwarrior would be pulling down a Sergeant's pay when they get to the unit-because they're a valuable commodity, but it also makes sense that the pay doesn't come with the duties. In the Old days of the army, Specialist rank went all the way up to eight or nine. NCO pay, but not NCO duties.
This, I think, is the equivalency most of the functionally effective armies in the Inner Sphere and Periphery would default to out of necessity, whatever else they might call it-for some, it may well be just a service so lacking in nco's and officer heavy that 'Sergeant" is sufficient, in others, there may well just be a "Mechwarrior" with an E-rating or equivalent, or some combination of those.
See, the basic difference between an Officer, and an NCO (in services with a strong NCO corps) is that an officer
Commands, while an NCO
Leads.
You might think those are synonyms, but no, they really aren't. An NCO's weapon is his Weapon. An Officer's Weapon is the men under his command, including NCO's. thus, why in the World War they gave NCO's submachine guns, because they were expected to use them, while officers got a pistol, and if he had to use the pistol, it meant shit had gone horribly wrong.