Butter cream icing substitutions for covering the cake with fondant

Butter cream consists mostly of almost water-free fat (butter or shortening) and sugar. Whipped cream has about 30% fat, the remaining part is solids and mostly water.

Everything that has significant amount of water will dissolve fondant since fondant consists of sugar. For example, cream, yoghurt, cream cheese, mascarpone and pudding (custard) are out. You have to use an icing with a very high fat content.

I am assuming that your buttercream is a meringue-based buttercream or custard-based buttercream (because you stated that you don't have much time to do a time-consuming cream). You could make a simple buttercream (also known as American buttercream) which basically consists of just almost water-free fat (butter or shortening), sugar, some flavour (e.g. vanilla extract) and a tiny bit water/milk. Making this buttercream is (almost) as fast as whipping cream.

A website (sorry, in German) says that you can use ganache, some cream creese icings with a high butter/shortening content (and white chocolate to increase the fat content), or warm jam (not cold jam due to possible condensation!). I don't know how high a "high butter/shortening content" has to be, so I would stick to the simple buttercream.