Other sources indicate a less innocent explanation: nudity is the penalty for not undergoing female genital mutilation. Amongst females, only those who've undergone it are allowed to wear clothing, and there are other consequences as well.
From BBC Tribe - The Dassanech
The Dassanech tribe is not strictly defined by ethnicity. Anyone – man
or woman - will be admitted, as long as they agree to be circumcised.
Dassanech girls are circumcised young, at around 10 or 12 years of
age. If they are not circumcised, a girl can’t marry and her father
won’t receive her bride-price, so he has a direct interest in her
going through the ordeal. Until they are circumcised, girls are called
‘wild animals’ or ‘men’ to tease them – the idea is that their
clitoris has to be removed before they act like women.
Girls may be circumcised in their mother’s house, or in another
village, but always with other girls of their age going through the
same ritual. The cutting itself is usually done by an older woman who
will be helped by the girl’s relatives. She’s held down, and a leather
strap is tied around her ankles or in between her legs. It is kept
tied to restrict the girl’s movement, until the wounds have healed and
the pain has subsided.
When the ritual has been completed, the girl is given sour milk to
drink and a necklace by her mother. From then on, she is allowed to
wear a leather skirt to show she is now considered an adult. Marriage
for girls often takes place soon after.
The Atlas of Humanity (about page, about page for DeFactory) and Wikipedia explicitly say that women who have not undergone FGM are not allowed to wear clothing, with pretty much the same text:
Women are circumcised by removing the clitoris. Women who are not
circumcised are called animals or boys and cannot get married or wear
clothes. Women wear a pleated cowskin skirt and necklaces and
bracelets, they are usually are married off at 17 while men are at 20.
Boys are circumcised. Men wear only a checkered cloth around their
The BBC article has no mention of nudity being the result of high temperatures.
While browsing the topic, it seemed like women who wore a skirt didn't necessarily wear a top. This may well be the case for the woman photographed, and may be the reason the photo is a headshot. I'd be surprised to see toplessness being interpreted as nudity, however. For an example of women wearing a skirt but not a top, from Getty images: (warning: photo of topless women) Topless dassanech tribe woman, Omo valley, Omorate, Ethiopia