Technically, it's not incest. There is no genetical similarity between the child and the surrogate mother. The only relationship between the child and the surrogate mother is the latter gave birth to the former. As soon as the child is born into this world, he/she is given back to its intended parents, who made up her genetical identity.
Is it acceptable though? I'd say yes, as long as there is no deeper relationship between the child and the surrogate mother from the start (like if the biological parents also allow the surrogate mother to raise their child for some time for whatever reasons).