In the mapping.xml you can add the following attribute to the asset_state field: skip-update="true"

Ex:

<field resource="in_service" source-type="boolean" target="asset_state" target-type="string"  skip-update="true" target-header="Hardware"

This will set the asset state when the asset is created in Freshservice. When an asset is updated, no change will be made to the asset state.