As I understand it, ashes are a sacramental, but not something that laymen are positively forbidden to touch.
Holy Communion (both species) and Holy Chrism can only be touched by a deacon (or higher).
What sacramental grace is received when laymen administer them is a mystery to me.
I guess it would be like your grandmother sprinkling you with holy water, vs. a priest sprinkling you during the Asperges Me. The latter carries more grace, as it is connected with the Church (and hence with Christ) whereas the grandmother only is applying a sacramental. Both are sacramentals, but when the Church/Liturgy is involved, they are more powerful.
Matthew