Yes. Children with Mosaic Down Syndrome may not have all of the physical characteristics common to Down Syndrome. In Down Syndrome, the defect of the 21st chromosome is present in all cells. In Mosaic Down Syndrome, the defect is not present in all cells. The degree of developmental disability varies depending on how many cells have the altered 21st chromosome and where those cells are.
There is also a condition called Williams Syndrome where individuals have similar facial features to Down Syndrome. Although developmental delays are common in young children, a significant percentage do not have mental retardation.