Mecha is a theme^A) focused on giant robots fighting (each-other, usually). Although this may sound controversial on the surface…mecha is most comparable to Isekai. Why? Well, just like Isekai, Mecha is so vast and varies so much that it’s hard to pin down much of anything that makes them similar. Besides ‘robots’.
I mean, if you want to see just how much mecha can vary…just look at the list of articles below. Each show varies in tone, subject matter and whatnot. Even their approaches to mecha (realistic/fantasy) isn’t the same.