That's because she is born as an Elemental. Elementals are decent beings and look like humans. Thanos was born with a Deviant gene. Deviants and Eternals are always at war. That's why Thanos looks weird. Thanos's parents are also Eternals so Thanos being a deviant was an exception.
Eternals and Deviants both came from Celestial beings. Marvel already introduced a Celestial in the MCU - Star Lord's father in GOTG Vol 2. Star-Lord got Celestial's powers for a while but lost them when he killed his father and destroyed the planet. Celestials are older than Odin and Asgardians as well. They are basically Marvel's version of Gods.