Yes, it is absolutely fine to put fondant icing on a fruitcake. Fondant icing can be used to cover and decorate various types of cakes, including fruitcakes. The smooth, pliable nature of fondant makes it a popular choice for creating a clean, polished look on cakes.
When applying fondant to a fruitcake, it's important to ensure that the cake is properly prepared and covered before adding the fondant. Fruitcakes tend to be dense and moist, so it's a good idea to brush the surface of the cake with a layer of apricot jam or a simple syrup to help the fondant adhere.
You can then roll out the fondant to the desired thickness and carefully drape it over the cake, smoothing it down gently with your hands or a fondant smoother. Trim off any excess fondant and use additional decorations if desired, such as fondant shapes, flowers, or piping.
Remember to consider the taste preferences of your guests since fondant icing has a distinct texture and taste. Some people enjoy the combination of fondant and fruitcake, while others may prefer a different type of icing, such as buttercream.