As it stands, biomodding is offered exclusively to corprorate citizens, with some limited exceptions. I think this is pretty appropriate. High-end cosmetic mods are pretty sophisticated work and they're a luxury that fits more into corporate society than they do the Mix. They also take a lot of time and effort to generate to it makes sense that they're offered selectively.
However, their selective availability has kind of drifted into some uncomfortable RL analogs to issues like gender expression that I think are mostly a byproduct and not intended through theme, but nevertheless, it makes things a little uncomfortable to negate.
The current implementation also leaves out what I feel is one of the cooler avenues for modding: less sophisticated, dirtier and more primitive flavors of body mod like the tooth bud implants the LoTeks use in Johnny Mnemonic and other black market, bootleg biotech.
I'd like to suggest adding a Mix bodymod clinic formally, bu onet that offers a different style and flavor to what they provide that will be more at home in the Mix. The process is more traditionally surgical than what's offered Topside, success rates aren't stellar and generally things are grittier. Swap your eyes out, get some cheap cosmetic chrome. Get some dog or cat fangs or rudimentary full conversions. I think it's a thematic avenue to give people more options to modify themselves in the Mix while respecting the boundaries of what's available to the slum and keeps things cpaf.