With Lightroom Classic we have option to sync the images through collections and we can turn off sync for a particular collection if required. When you import the images in Lightroom and enable the sync in Lightroom Classic, it syncs all the images available in the cloud storage. This is by design and we do not have the option to choose which images get synced neither do we have the option to disable sync from Lightroom to Lightroom Classic while syncing/uploading images from Lightroom Classic to the cloud.
You can remove them from the mobile download.lrdata to save space on the machine but the images would then show missing from the Lightroom Catalog. Also, Lightroom may try to re-sync the images to ensure that the number of images in the cloud storage matches the number available locally to Lightroom Classic whenever the sync is re-initiated in future.