This question touches on one of my biggest pet peeves, and it is such a huge pet peeve because it makes finding information incredibly difficult for people like you – when people conflate fabric type with fiber type. So first, a short lesson in fabrics, so that you know why people’s recommendations on this are not very good.
When talking about fabrics, there are two main properties that are discussed. One is what the fabric was made out of, and the other is how the fabric was made. The former is referred to as the fiber, and the latter is referred to as the finish, the fabric type, the weave (if it’s a woven fabric), etc. “Cotton,” in this case, is the fiber, but it tells you nothing about what was done to that fiber in order to turn it into a fabric – this would be akin to someone telling you “plastic,” but not specifying if that plastic had been made into a spoon or into a bottle. (Similarly, a spoon – analogous to the finish here – can be made out of many different material types, just like how a particular fabric weave can be made out of multiple fiber types.)
Now, for the answer: cotton is a good fiber to look for when making a lolita-style blouse, so that half of the recommendation is good. Other weaves are also used (chiffon being one of the more popular ones), but for a basic blouse, you are mostly going to be looking at cottons and cotton blends.
When people refer to just plain “cotton,” they are typically talking about a flat weave sheeting. This would be the type of fabric you would find in varying weights and colors in the quilting section of the fabric store, and would be a decent option if you found one with the correct weight for a blouse. I would stay away from the cheapest options and anything that looks too coarse, and instead look for something that has a smooth finish and is medium weight.
I have, in my lolita fashion days, used heavier cotton muslins to great effect, as well. I would recommend that if you are looking to make a white or off-white “natural” blouse.
Cotton or cotton blend sateen would also be a good option if you want something with a little bit more sheen to it, but keeping the same texture. It would be good for a slightly more formal blouse, especially in gothic or classic style. You may find this in the quilting section or in the sportswear section, depending on the fabric store and the weight of the fabric. Sateen is basically the same weave as satin, but it is made from spun fibers (like cotton) rather than extruded fibers (like polyester or silk) so that it has less of a shine to it, fyi.
Another good option would be a cotton poplin, which is often used for dress shirts and has a slightly corded texture to it. If you don’t mind the blouse being slightly sheer, a cotton batiste would also be an option.
Most of these fabrics also come in stretch varieties, which aren’t going to be as stretchy as a knit, but have a bit of spandex fiber mixed in to allow the material a bit of give. This helps with comfort when wearing, so don’t be afraid of these. You may also come across cotton/polyester blends, which are good when you want the fabric to resist wrinkles but still have the overall look and feel of a 100% cotton material.
When you’re in a fabric store, an employee would be able to point you towards any of these materials if you end up not being able to find them yourself. I would recommend touching all of the fabrics and draping them over your hand to make sure it’s what you want before buying.
I hope that helps! Good luck. :]