Of all of my favorite spicy cuisines (Mexican, Thai, Szechwan, and Indian), Mexican is the one that is what I call the most "labor intensive". Of course, there are many simple dishes, but most of those dishes based on the dried chile peppers are not fast to make, though a blended makes them much faster, for sure! Imagine grinding all of that stuff on stone! However, it is still doable, for any of us who love to cook.
Unfortunately, unlike recipes with cayenne pepper, or crushed red pepper, that just add heat, you can't just reduce the number of peppers to make it milder to taste, as the flavor is based on the chiles. Some of the commonly used chiles are fairly mild. Anchos (the most commonly used dried chile in Mexico - a ripened, dried poblano) and mulatos are very mild - 500-1,500 Skoville Units (SUs). Guajillos are another common chile, which are a little hotter - 1,500-2,500 SUs. Pasillas are the long, almost black chiles, which are up to 4,000 SUs, but usually milder. Cascabels are a round chile, that is usually around 5,000 SUs, but sometimes up to 10,000. And, as Kayelle noted, removing the seeds and veins will reduce the heat considerably. However, with fresh chiles don't scrape out the seeds and veins - this breaks up the cells of the veins, leaving a lot of the oils on the inside of the pepper. Instead, take the tip of a paring knife and cut the vein out, breaking it up as little as possible.
Here are some peppers to avoid, if you want mild. Not only are they hot, but not really things you can remore the seeds and veins from! Chile de árbol is the one that looks sort of like a Thai or Chinese type pepper, and really doesn't have a lot of flavor - just heat! It's around 30-40,000 SUs. In recipes with multiple varieties, when you see these, they are just for heat, and you won't miss them. And where these are the only chile for the flavor, there are going to be a larger number, and definitely a hot salsa! Serranos secos are a ripened, dried serrano, shorter and fatter, and a little milder - 15-20,000 SUs - so you could substitute those, but again, they are usually more for heat than flavor. Chipotles are a favorite of many, and the dried ones are the best flavored, but not mild! The chipotles mecos are the tan colored ones, and are around 10-18,000 SUs, but I've had hotter ones! And the moritas - the smaller, almost black ones - are even hotter, though the best flavored. But if you are looking for a dried, smoked pepper, that is milder than these, Pasilla de Oaxaca is one with great flavor, but the mildest smoked pepper I've tried, though it's also fairly expensive. The size is large - about the same as mulatos. And for canned chipotles, they can also be quite hot, depending on the brand. When I was first testing out all the brands, the one that was the mildest, but had good chipotle flavor, was San Marcos - the one with the blue label. Herdez was the best flavored, but the hottest, as well, always with a morita in each can. Goya didn't have much flavor, while La Costeña, and La Morena were good, but definitely hot.
For the fresh peppers, poblanos are the mildest, and easy to remove the seeds and veins from. Jalapeños are the most common, but, unfortunately, are unpredictable! They can vary from 0 (only if you grow it, for the most part) to 6,000 SUs, some hybrids even more. I have actually seen (only a couple of times) TAM jalapeños (a variety of mild jalapeños) in the supermarket, but usually they just say "jalapeños". Serranos are hotter, but usually, not many are used, and the seeds and veins can be removed. There are milder varieties, but not in the stores. And, of course, you'll want to avoid habaneros! However, if you grow things at home, try some Aji Dulce - this is a type of habanero that is around 500 SUs, or less, and gave me a way to share many of those delicious habanero based dishes with friends, who could not even sample them before.