Yes, they still exist in Hueco Mundo. Mayuri has been there a few times since the defeat of Aizen, but there is no reason to suggest that he has somehow driven all Hollows out of Hueco Mundo. The might hide while he is around, but at this time as far as we know, they are still there Las Noches is the citadel inside Hueco Mundo, just like Karakura Town is in the Human World, or Seireitei is in Soul Society.