IMO, excepting boss characters like Shao Kahn and Goro the primary characters of Mortal Kombat as a 27-year franchise are:
- Scorpion, Sub-Zero, Raiden
- Liu Kang, Johnny Cage, Kitana, Mileena, Sonya Blade, Jax
- Kano, Shang Tsung, Kung Lao
- Reptile, Baraka
Roughly in that order (and ordered within tiers). It's the MK1/MK2 playable cast, arranged from most recognizable to everyday people who aren't MK fans to least recognizable unless you're a fan. In particular, anyone who was in the first Mortal Kombat movie, which was a massive worldwide blockbuster film that is most people's only actual exposure to Mortal Kombat, is iconic to the franchise.
After that, I'd say the following characters are the next tier down:
- Jade, Smoke, Noob Saibot, Cyrax, Sektor, Sindel, Kabal, Nightwolf, Ermac, Kenshi, Quan Chi
Those are all characters it's nice to have, but I'm never super surprised if one of them doesn't make the cut.
Nobody argues Street Fighter should have Guile, Cammy, or Chun-Li sit out because there are 'too many special forces characters'. Those characters are iconic core figures in the franchise and at this point any iteration of the series is going to feature them. MK similarly trafficks in a pop culture brand that is nostalgic and iconic, and if you start dropping the characters most identified with the brand, the general public will lose interest.
We are already seeing Cassie Cage and Kotal Kahn in particular achieve this status with a new generation, btw -- I would be surprised if either of them goes away anytime soon.