Respect, respect, respect. I can't repeat this too many times.
If you want to sew Native American doll clothes, you need to put research into it and make sure that whatever you sew is respectful and accurate. There are far too many people who do not do this.
Native American tribes are not interchangeable and should not be treated as such.
If in doubt, err on the side of leaving it alone.
There are also only a few patterns out there for sewing Native American doll clothes.
There is a pattern for sewing a dress similar to Kaya's meet dress here.
Wren*Feathers also has free patterns for sewing tribal costumes for the Crow and Tlingit nations here. A pattern for a Navajo blouse and skirt is here.
Wren*Feathers also has a great pattern for a Jingle Dancer Powwow Dress.
The Wren*Feathers patterns are great examples of how to do it right! They are respectful, well researched, and distinguish between individual tribes.
This and this and this are very good examples of what NOT to do. They are not specific to any tribe, they just show a stereotype of what people think a Native American tribal outfit looks like.
This tutorial from Pixie Faire on how to make a pattern from All Dolled Up into a faux-Native American dress is so disappointing, especially as people have written them to tell them it is wrong. It's especially gross that they are using Kit as the model.
Another example of what not to do is the Read Creations Native American dress.
Even with patterns, I recommend researching before you start sewing.
If you are lucky enough to visit the Field Museum in Chicago, you can see a wide variety of tribal costumes in the Ancient America exhibits, and you can see how different traditions can be. The Autry Museum in Los Angeles is another great source of information.
Obviously, not everyone is able to visit these museums or a similar one, but read a book! Your local library will be happy to help you.
Be careful when looking at resources online, and make sure that they are reputable. For example, you can search the Library of Congress for historical photos, which would be a reliable source of information.
DO NOT use Native American clothing or interchangeable, buckskin "costumes" as Halloween costumes.
These are important parts of culture, not costumes for amusement.
Overall, Native culture is so important, I think it is best to abstain from sewing Native doll clothes without a lot of research.