Brian Van Fleet is correct.
In bonsai, the answer to every question is “It depends”. But typically, the soil should be level in the pot, just under the rim. Not mounded. With moss added, that puts the moss level with the rim.
Notable exceptions are cascades and semi-cascades, Shohin, and bonsai planted on slabs.
Even on Shohin, it’s possible to have level soil:
Do a google search on “Kimura bonsai”, then click “Images”, and scroll thru his trees. The vast majority of trees in pots will have level soil.
Anyone who tells you “Oh, it’s ok, just do what YOU like...” isn’t teaching you good practices . Sure, it’s more difficult to create a proper rootball to fit into a bonsai pot. Bonsai as an art is a challenge. It’s details like this that separate the good bonsai from the mediocre bonsai.