is she that popular in NMIXX?
51K is a lot for a new soloist
Yes. She was probably the most popular internationally.
In Korea, she was like #2 or #3. At the time of debut, Sullyoon was the most popular, but then slowly Haewon took her place. During that time Jinni was there and before Haewon became most popular, Jinni and Haewon were hypothetically battling for the #2 and #3 spots.
Also, each member of NMIXX could easily sell 30k with their China bars, so I think 51k looks about right factoring in her Chinese, Korean, and possibly international fanbase.