About what I read, Yocto and DietPi are quite different in their aims. Yocto for me sounds a bit similar to the “Tiny Core” project, not so much with the “tiny” aspect, but the adaption and customization down to the kernel/firmware level.
Please read the aspects that I wrote about “Tiny Core” vs DietPi, as most of them should true for Yocto comparison as well: https://dietpi.com/forum/t/diet-pi-run-in-ram/2355/2
Really the most important aspect about DietPi is, that you have a fully functional Debian/Raspbian system with all it’s benefits, using APT package manager with access to the whole software repo, having all the various guides, man pages etc. that fit as well for DietPi etc.
So DietPi is not a minimal, IoT customized Linux distro, but a minimal SBC optimized Debian/Raspbian distro.
Also about the “learning curve”. The goal of DietPi is exactly to offer software, config etc. to less experienced users as well, by offering the GUI and automated install steps, so you don’t need to know too much of the Linux depths. It was the goal from the beginning of DietPi 
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And this exactly is excluded from the other two projects, which more likely address Linux devs and coders with quite some insight into Linux systems and who are able to build software themselves from source, if not available pre-compiled for their systems.