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Literally meaning dried fish woman, the popular slang 干物女 is used to call a woman in her twenties or older who, as nicely summarized in Wikipedia, has many of the following traits:

メールの返事が極端に遅い、短い (Her text replies are very slow and short.)

簡単な食事なら台所で立って食べる (If it is something simple, she will eat it standing in her kitchen.)

忘れ物を靴を履いたまま、膝立ちで部屋に上がり取りに行く (She will go to take forgotten stuff in her flat on her knees keeping the outdoor shoes in the air rather than put them off.)

休日はノーメイクでノーブラ (She won't put on her makeup and bra on nonworking days.)

半年ほど美容室に行っていない (She won't visit a beauty salon for half a year.)

冬場は毛の処理を怠る、又はしない (In winter she won't shave or wax her body hair properly or at all.)

1人で居酒屋に入れる (She has no problem going to a pub alone.)

最近ドキドキしていない (She rarely gets so excited that she experiences an increased heartbeat.)

What is the German equivalent? In other words, how are such women commonly or idiomatically called in German?

In short: There is none

At least none which barely covers more than two or three of all these aspects.

But, as japanese wiki says it's a term originated from the manga Hotaru no Hinari by Hiura Satoru. Some japanese words already made it into German youth culture through manga. They are not common, but words like kawaii are known + used by fans and are sneaking into our passive vocabulary. Even translators of mangas and animes are starting to add -chan, -kun, -sama, etc to names. Things which were omitted a few years ago.

So ... maybe in a few years (and when this manga/anime is available in German) there will be the japanese word, or it will be adapted from the english translation.

EDIT:

Please note, that other terms for people like Otaku おたく/オタク or Hikikomori ひきこもり aren't translated as well, when there is no (western) equivalent behaviour covering all aspects.

This is a very pejorative use of imagery already at the Japanese source, it seems. As such it is quite specific to Japanese culture as well. A direct translation or one-word translation or even a very short combination in German seems to not exist.

If one equivalent catches up in German it will probably be a loanword from manga-Japanese?

The fish-part especially seems to give it a twist that together with the list of traits ascribed to it is a bit too unique for a direct fit.

As the Japanese, Chinese and Korean Wikipedia entries – which are the only Wikipedia languages having an entry for this concept – explain: this is a description of women who 'lost love' or

The dried fish woman ( Japanese : dried woman Himono Onna [ * ] ) is a Japanese coined word meaning a woman who abandoned love.

The dry woman comes from a Japanese buzzwords and is used to describe a young woman who is as dry as scallops and mushrooms. It refers to a group of women who can't afford to love and think that many things are very troublesome and only have to be done. This name is derived from the Japanese name for dry fish, and the words that are close to 'funeral' and 'defeated dogs.'

_{(Machine aided translation, please excuse me, and please improve if you can)}

Which I would take to mean that these are relatively young women, disillusioned with the marriage market and societal expectations, especially for private relationships.

There is a very close combination of words in German which expresses a similar concept:

vertrocknete Jungfer (literally 'dried-up maiden') – but this is without context or explanation one standing idiom to refer mainly to somehwat older women, not those in her twenties, and a few other mismatching connotations, among them the pejorative undercurrent.

Jungfer f. today only in psychologically evaluating sense for '(older, unmarried) sensitive, prudish woman'. Mhd. juncvrouwe is shortened to late juncfer (14th century) by concentration of the emphasis on the first compositional element and applies (like Jungfrau (virgin), s. ?jung) to the unmarried lady of knightly rank, soon becomes the name of the unmarried daughter, later the unmarried bourgeois woman (also in the salutation) as well as the sexually untouched girl, cf. deflower, virginity (17th century). An disparaging sense developed alongside this already in the 17th century (young 'prudish, squeamish', old maid). The meaning 'servant, maid' (preserved in Kammerjungfer, 15th century) comes from the custom that young girls come up to noble ladies. Brautjungfer (Bridesmaid) f. 'single friend or relative of the bride as companion in the wedding procession' (18th century).

The psychological background given in your comments could be regarded as »Resignation«. The dried fish would therefore be

eine Resignierte (nominalized from »resigniert«)

but this is no common expression.

Related term: »innere Kündigung«