Kittl is a design tool with an opinion. It is built around typography, vintage styling and print-on-demand, and the editor is noticeably stronger than Canva's for that kind of work. Mockups are there to present the finished piece.
Canva is a general purpose design tool that happens to include mockups, and since it absorbed Smartmockups it inherited a large and well liked scene library. It is free, it runs everywhere, and most people already have it open.
The real split is pricing model and depth. Kittl meters AI in tokens, 200 one off on Free, 2,000 a month on Pro at $15, 6,000 on Expert at $30. Canva's mockups are available on the free plan. So Kittl asks you to pay for a better editor, and Canva asks you to accept a more general one for nothing.