I'm finding that unless I manually manage the military appointments, there tend to be far too many low ranked people compared to the higher ranks. With that in mind, I've got a couple of suggestions:
This is because the promotion system was changed to an on-demand system in 2.0, due to the numerous challenges of a fixed rank ratio system. If you want more promotions to higher ranks, you need to create more higher-rank positions to be filled. Otherwise, there will not be automatic promotions if there is no job for the person being promoted to fill.
If you have too many low-ranked officers, you can either create higher-rank commands to encourage promotions (you can use the "Senior C.O." checkbox in ship design to help diversify your ranks) or use command modules (AUX, ENG, CIC, etc.) to give your low-rank officers more things to do.
Note that once you get to the flag officer ranks (CDRE/R4, RADM/R5 or so) this means you will have to create more admin commands at the appropriate level. Depending on your rank structures, this may mean having a finer-grained command structure or having "superfluous" commands for roleplay. One idea is to have NAV admin commands which prioritize Crew Training, Reaction, Engineering, or Tactical differently rather than just a general-purpose NAV command at each level. You can also play around with the range of the admin commands, e.g., you might have several SRV commands located in different systems to provide bonuses in several areas of operations.