How can I make my Chocolate Mousse fluffier?
The dessert pictured above definitely, definitely has gelatin in it.  That will change the mouth feel on the mousse slightly, but it might be what you are going for and will provide some of the stability that you are looking for.  To have it hold a form like this you will need a recipe that requires gelatin; I would recommend gelatin sheets if you can find them at a local baking/cooking supply store or order them online.

The fluffiness will also be determined by several things.  How were the whites whipped (too much, too little) or how was the cream whipped (again too much, too little)?  There are recipes that call for both.  How you incorporate all of the ingredients into one another really matters and in what order, this is called tempering.  You want to fold until the mixtures are just combined; no more.

Temperature is of key importance for all of the ingredients - some you want very cold while others (ie chocolate) you want to be room temperature but not too cool or it will seize up the mixture and the mousse will taste grainy.

Also chocolate mousse texture can definitely be affected by the percentage and quality of chocolate you are using.  If is is of a higher percentage it can create a denser mousse and deflate the whites and cream more easily.