A "fruit pizza" is just a cookie crust fruit tart shaped like a pizza rather than formed and baked in a tart ring. So any tart filling will work fine with your cookie "pizza" crust.
Buttercream would be tooth-achingly sweet as a filling.
Since you will use whipped cream, I assume dairy isn't the issue, and that its just cream cheese you don't want to use. So the standard tart fillings will work.
Classic pastry cream (the standard for fruit tarts)
Yogurt pastry cream (lighter and better for fruit tarts)
Dairy-free pastry cream
Fruit curd: lemon, passion fruit, mango, lime,
If there's concern about lactose, Green Valley Organics makes lactose free yogurt and cream cheese.
Unripened cheeses like cream cheese, ricotta, and cottage cheese all have 5% or less lactose.
Lactose is concentrated in the whey. Most cheeses and cultured milk products have very little whey. Aged cheese is essentially lactose free as all the lactose disintegrates during the aging process.
Being lactose intolerant myself, I can eat most yogurt. Unripened cheeses, kifer, and sour cream in moderation. But when it comes to milk, I can't drink anything but lactose free.