I tried to replace 2 cups of icing sugar with corn starch but my fondant is crumbly, and not smooth at all when i roll it. Is there a way to make it not so sickly sweet?

I don't think there really is. It's a sugar dough. You may be able to add a little salt; I know lots of ABC users do that for the same reason.

If your recipe calls for water, maybe try replacing some of it with lemon juice.

It's made out of sugar. It's supposed to be sweet.