By meringue frosting, do you mean Rose’s Light Italian Meringue recipe?
In general, I think it’s probably a good idea to keep dairy (like cream cheese) and eggs in the refrigerator until a few hours before serving.
That said, I’m wondering if the main problem with leaving meringue out is that it will continue to attract moisture from the air. The other issue may be sugar content. A high saturation of sugar creates a hostile environment for microbes, for example, the Royal Icing recipe will keep at room temp for three days. The Light Italian Meringue has the minimum amount of sugar that will still stabilize the foam and so may be less hostile to bacterial growth.
Mousseline has all that butter, which works to moisture-proof the meringue.
As for cream cheese frostings, Rose’s white chocolate cream cheese frosting is good for 1 day at room temp. If you are looking at a recipe that doesn’t contain white chocolate and lemon juice, then it may have different keeping qualities.