MAAS only supports custom images which are DD(full disk image) or tar(archive containing all system files). In either case the custom image must have all all files required to boot in the image. For Ubuntu images coming from images.maas.io we ship the base OS image as a SquashFS file with the kernel and initrd as separate resources. This allows MAAS to make the Ubuntu images available for PXE booting in addtion to deployment. We do not currently support SquashFS images for custom image deployment nor do we support PXE booting images that do not come from images.maas.io.
For Ubuntu customization I suggest using cloud-init or preseeds.