Engineers who design tall buildings use top-down design. This is because they must know how much the upper floors weigh and how those floors are built along with any other special considerations. Only then can they begin to design the next lower floor that will support everything above it.
A mobile might be designed from the bottom up if the weight of the lower parts became a concern. Fortunately CD's and sticks don't weigh very much! However, you might want to design your mobile from top to bottom if you want it to be, for example, symmetrical.