Is dietpi good for iot?

Is dietpi good for iot? and what about safe remote upgrades? are they possible?

Well this depends on your SBC and use case. DietPi is not an own Operating System or Distribution. DietPi is a bundle of scripts on top of a standard Debian installation. Nothing more. For Raspberry PI devices the base image is Raspbian Lite. For other SBC usually Armbian. The whole software update process is done the same way as on all other Debian distribution.