Place the yogurt, crème fraîche, lemon rind and sugar in a large bowl and beat together with a hand-held electric whisk until smooth. Add the lemon juice and whisk again until the mixture has thickened slightly.
Whisk the egg whites in a clean bowl with a hand-held electric whisk until they form soft peaks, then fold into the lemon mixture.
Spoon the mousse into 4 glasses and chill for 10 minutes. Serve decorated with the lemon rind.