It's perfectly possible to map the shops in an indoor mall, see e.g. https://www.openstreetmap.org/#map=18/51.87918/-0.41423 . You're right we can't use the Mall's own map, but (Covid restrictions permitting) it's straightforward enough to map the shops manually by surveying them on the ground.

I don't thing there's an standard practice to deliberately leave out smaller shops, though it's perhaps inevitable that larger stores are more obvious, perhaps have outside entrances, and so are more likely to  get added first.

The density isn't an issue since map renderers already have mechanisms in place to avoid overlapping text and symbols.