Attack on Titan is for all intents and purposes a mecha, and one of the most mainstream anime of recent memory.

It has mecha elements, as well as steampunk and magic elements too.

Other than their kits, everything else they operate is very standard WW1-esque tech.

Gundam, the niche that does 100 billion yen in revenue per year.

The toys are appealing even to children that never saw the anime.

Actually I wouldn't let my kids watch it until they were at least 13 or so, maybe more depending on the child. A lot of war and mass-scale death there, and frankly, a lot of adoration for fascism - the space nazis turn out to be right.

Wing (and, by extension, 00) was a fine watch at 10, and G probably would have been, too. They're considerably less fraught in both iconography and philosophy than some of the other series, and quite a bit more hammy. The Build Fighters/Divers series are great, too. Stay far away from AGE, though; cutesy stylings because it was intended to exist alongside series like Yokai Watch, but it ended up being quite problematic.

Anime and Fascism really do seem to walk hand-in-hand a lot.

Even my beloved Frieren has an unnatural dehumanistic streak against Demons who exhibit intelligence and personalities, which she explains away as mere mimicry, despite the fact that she has learned magic from them.

I guess I know which side of the AI debate she rests on.

Mecha fandom doesn't necessarily cross over with anime fandom (even though the former is arguably the origin of the latter). I find that there's more crossover with hobbyists (plastic models), military fandom, and gamers. Gundam is almost it's own thing (to Sunrise's simultaneous satisfaction and chagrin), and the Site Which Shall Not Be Named split a Mecha board off of the Anime board almost 2 decades ago. I don't know that the fandom will be represented on MAL, which is itself a niche service for the wider anime-watching community.

Suffice it to say, the multiple life-scale Gundam statues that have been put up/dismantled over the past few years speak to its enduring popularity. I will say that there haven't been as many non-Gundam mecha anime as there were in the 2010s. We did get G-Witch and Bravern last year, and Hathaway on Netflix in 2021. And, as someone said, AOT is essentially Meat Gundam.

EDIT: Also, I don't know that I'd even call 86 mainstream. It felt like it actually got way less recognition than it should have.