Here's one that I modified from another recipe on the site. I don't think it's too sweet, because I also don't like super sweet. To be sure, add the vanilla at the end 1/4 teaspoon at a time until it's a sweetness you like.
One 16 oz jar of marshmallow fluff
½ cup shortening (non butter flavor)
2/3 cup powdered sugar
¾ t. vanilla extract
4 t. VERY HOT water (this helps to make the fluff easier to spread)
¼ t. salt
9 oz (1/2 tray) Oreo cookies
Instructions
Combine marshmallow fluff, shortening, powdered sugar, salt, and vanilla in a medium bowl and mix well with an electric mixer on high until fluffy. Add the hot water to the fluff mixture mix on high for 30 seconds or so until combined. Fold in cookie pieces*
*if using a stand mixer with paddle attachment the cookies can be put in whole, the mixer will break them down into smaller pieces.
I just made one the other day that is my new favorite filling period. It's basically a french cream with crushed oreos.
1c milk
5tbsp flour
1c granulated sugar
1c butter, half salted half unsalted, softened
1tsp vanilla extract
1/2c crushed oreos
Whisk together the milk and flour over med heat until milk bubbles and thickens to a pudding consistency. Remove from heat and cool to room temp.
Beat butter and sugar in stand mixer on med-high speed until fluffy and most of sugar has dissolved. Blend in vanilla. With mixer on low slowly spoon in the cooled milk mixture. Beat on med speed until peaks form. Fold in oreo crumbs.
Sooo yummy, and not too sweet. 
HTH!
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