Musings on clothes
General Attire

Notes
Most of the base game spirits have this in common: A shoulder piece with a triangle-lined trim, and a diamond symbol; and garments that are generally loose/not form fitting. This pattern of clothing will be referred to as "standard attire".
Spirits wearing standard attire have little deviations in how the common patterns are styled.
In previous versions of the game, all spirits with a shoulder piece (with the exception of some spirits from Valley and Wasteland) had it so the item draped over their shoulder. This would suggest that the item was made with a more flexible material. In the current version of the game, these same spirits now have more rigid shoulder pieces, somewhat akin to the aforementioned Valley/Wasteland spirits. Spirits who still have the flexible version of the item also have it extend to be a hood over their head.
Lots of variation in sleeve and garment length. For spirits with short garments, one can see shorts-like clothes underneath.
Standard attire seems to be worn in a lot of contexts. The attire of a candlemaker or a soldier is very similar, if not the same in some cases.
There are other clothings which occupy a general, less specialized context but are less common than standard attire. This includes the outfit worn by spirits like the Blushing Prospector, Dreams guide, and Rhythm guide.
Athletes and Warriors

Notes
Standard attire appears in both contexts, however these kinds of occupations feature more specialized clothings.
Athletes and warriors both feature a major deviation in standard attire, with large, rigid shoulder pieces and belts to keep their garments closer to their body, possibly for ease of mobility.
Both athlete and warrior attire features head pieces which frame the face, though they likely vary in purpose and function. For the athletes, the head pieces combined with their hair style and capes help to signal their strength, reputation, or wealth. General opulence and valor. For warriors, their head pieces can be seen covering more of their face, almost like a knight's helmet, so it's likely they are more for genuine protection.
The head pieces of athletes and warriors also differ in material. Athlete's are a silver color, while warriors are golden.
The similarity between some athlete and warrior attire could signify some relation between the Valley of Triumph and the Golden Wasteland. Not only are the realms neighbors, but perhaps during the Last War (or other conflict) the valor of the Valley's champions would funnel some of them into becoming the Wasteland's soldiers.
More on warriors
With the release of part 1 of Sky: The Two Embers, it has given more specific insights as to what warrior outfits looked like before everyone became spirits. As of writing, it appears these details are specific to warriors.

Attire seems matches how it appears in-game with other spirits aside from the Scarred Sentry, but also has it's own deviations.
In the show, there seems to be two layers of shoulder armor instead of one. Both layers appear rigid.
There are now fuller helmets covering the entire head. It seems like if the soldier had any hair beneath their helmet, it was tied up outside.
It might be more practical to just shave the head, but perhaps this method was done to bolster a sense of pride and aesthetics.
Variations in uniform color, seems to range from blue, to green, to a yellow-ish green, and purple. These colors could signify a warrior's designation/rank in the army, but it is not touched on in the show.
Some warriors had capes. These capes are much simpler in construction compared to the capes seen on other spirits, probably so that the capes don't impact a warrior's performance.
Instead of a diamond on the front of the shoulder piece, there is instead a symbol depicting the head of the King.
The Scarred Sentry's outfit has a sword hilted on its side, though in the show and in the game warriors are more often depicted using spears and shields. It is possible swords were used in daily enforcement and close combat situations, rather than for dealing with large threats.
Mages/Scholars/Clerics

Notes
In the game, all mages feature an enlongated version of standard attire. The garment reaches to the floor, and their sleeves are long enough to cover their hands. Given the less physical nature of their work, the impractical aspects of this clothing choice aren't as important.
In the game, a majority of the spirits in the Vault also have some sort of crest on their head. Some are gold with etching, while others are plain and black.
The golden crest could be a sign of status, as it is mostly seen on those with higher magical ability (such as the Levitating Adept and Lamed, the Vault Elder). Additionally, in the Two Embers Part 1, Lamed is seen using the diamond on their crest to read the mind of the Child, further hinting at high magical prowess.
A possible purpose of the etching seen on the gold masks is for identification, similar to dog tags. Especially in the twilight days of the kingdom, immortalizing personal history could have become much more important to workers of the Vault.
The black crest could be used to mark those with emerging, but not profound magical abilities.
In the Two Embers Part 1, the people working with Lamed do not have a crest, instead all having some form of hat and a hood to go over it. It is possible that these people are not particularly involved with magic. They are depicted maintaining memory lanterns in the vault, though they are more often depicted interfacing with the sick and tending to the dead. Because of this, this line of Vault worker could be referred to as "clerics".
Clerics are almost always seen with a shoulder accessory similar to the one worn by the Remembrance guide. In some instances in the show, one can see two little buttons on the front that keep the two ends of the sash together, likely so they don't slip off the shoulders.
Lamed has a piece of cloth running down the front and back of their body which is similar to the sash worn by clerics, however it is distinct in its patterning and it's color (purple vs. blue). This could signify that Lamed's line of work is different from the clerics, or it could be another signifier of rank or importance.
The robes worn by Lamed and the clerics in The Two Embers are white, while in the game it could be presumed that the robes for the spirits were some shade of grey.

A living ancestor from one of the Two Embers seasonal quests. They share the same model type as the mages.