Is there a frosting that's sweet like American Buttercream but textured like Italian Meringue Buttercream?
My wife uses whipped cream frosting, which is quite smooth and soft. 

Basically it is cream-cheese frosting with whipped cream added in, and then beaten with a wire whisk beater (easiest to do in a cake-mixer).

A basic recipe is:


8 ounces cream cheese   
1 cup powdered sugar
2 3/4 cups heavy cream, cold


Instructions:


Place the cream cheese and powdered sugar in a large bowl and beat
on medium-low speed until smooth.
Turn the mixer down to medium-low speed and pour the heavy cream
down the side of the bowl in a very slow and steady stream, stopping
every now and then to scrape the bottom and sides of the bowl with a
silicone spatula.
When all the cream has been added, turn the mixer up to medium-high
and whip until the frosting holds stiff peaks.
Flavor it as you need, folding in the flavoring


You can sweeten any frosting fairly effectively with extra sugar or sweetener, however too much sugar will cause any frosting to collapse as it absorbs water from the air and other ingredients. If you want to make it extra sweet, use an artificial sweetener