Having lived in India, China, and Taiwan, and having Mexican in-laws I hate to break it to you, but there is no "right" levels of heat for the cuisine. Putting aside regional variations, there is still a wide variety of personal preference. From my personal experience, Korean food has been consistently the spiciest. But that is based on only one trip to South Korea and anecdotal stories from a business partner who didn't drink alcohol and worked on increasing his tolerance for spicy food to fit in when he lived in Korea. I've also found most dishes in Korean restaurants in the US, Taiwan, and my one trip to Korea to have a lot of heat.
In India it depends on the dish and preferences. But I spent most of my time in living in Maharashtra. Which isn't known as having the spiciest cuisine in India. But when people there order spicy, it packs a lot of heat.
In Taiwan, food tends to be a bit spicy. Some dishes are infamously so, such as Ma La Hot Pot. But usually, even in the night markets, you say how hot you want something when you order it. There are 5 levels: "wei la" (mild) to "te la" (heavily spicey)". Just saying "la" means spicy and you'll get it as spicey as the cooks thinks is basic spicy and that can change from restaurant to restaurant, food cart to food cart. I assume it is the same in China, but I never remember ordering with such specificity in China, either because my Chinese was worse or because I wasn't living in areas known for spicy food.
For Mexican, my experience was more from my best friend growing up, whose mom was mexican and ran an out-of-home mexican food ordering business, and my brother in law and his friends. My brother-in-law is probably not the best example because he is seriously addicted to heat. To the point that by his early 40s he was getting stomach ulcers and the doctors were telling him to cut back on the spice. But my sister doesn't have a high tolerance for really hot food and has never had an issue with typical dishes in Puerto Escondido, Oaxaca, where they live now. I assume the spiciness of Mexican cuisine varies by region. What has always struck me about mexican cuisine is not so much that it is hot, but how they tend to add a bit of heat to everything. Like even their lollipops are coated in dry chili pepper powder.
My last observation, and I don't know how logical it is, but it is how I think of it, I find Mexican and Indian spicey to be a "dry" spicy and easier to deal with. The heat tends to be added with dry spices or fresh pepper. I find Thai, Chinese, Taiwanese, and Korean to be wet/oily spicey, using a lot of infused spicy oils which can make for some seriously spicy dishes if you are not used to them. I've rarely had an Indian or Mexican dish that was too hot for me to eat. But I've had plenty of experiences with Korean, Thai, and Taiwanese dishes that were hotter than I could deal with.
Lastly, I'll say, while it seems most prevalent in the US in starting in past few decades, young men proving their toughness by the level of heat they can endure is an international phenomena and should not be treated as some litmus test on what is normal for that culture's cuisine.
