All languages have words that are unique and hard to translate, because most other languages don't have a perfect matching translation for it. The German word Gemütlichkeit is one of them. You can translate is as "cosiness, snugness, sociability, comfortability, warmth, friendliness, slowliness, homelikeliness, ..." but all of those words matches only parts of the meaning of Gemütlichkeit. Other German words that are hard to translate are fremdschämen, Schadenfreude, Nervensäge, Weltschmerz, Erklärungsnot, Mahlzeit!, Fernweh, Fiesling, Schönwetterfreund and some of them even are used a foreign words in English: doppelganger (from German Doppelgänger literally: double walker), kindergarten (Kindergarten = children's garden) and many others.
The adjective quirky is one of the English words that are hard to translate into German. Others are mind, awkward, doggy bag, no-brainer, serendipity, bromance.
All you can do with such words is to step away from the word-to-word translation level and enter the "meaning level". Some call it free translation. You have to analyze, what is the meaning and intention of the speaker or writer, and then you have to re-think this meaning in the other language and express this meaning and intention in the other language. If you have to express ideas, that are easy to express with one of the words discussed here in one language, you maybe have to build complete new sentences in the other language.
This is what makes translation so hard. Every language has a set of words, and this set limits the way of thinking. Other languages have different sets with different meanings. So, speakers of other languages have other limitation of how they think. (And multilingual people have less limits of thinking, because they have more possibilities to think.)
If you grow up in an environment, where being gemütlich (adjective) and Gemütlichkeit (noun) are part of your everyday life, and then you want to say: "Simon ist ein sehr gemütlicher Mensch" or "Dieses Café strahlt eine angenehme Gemütlichkeit aus." or "Ich trinke erst mal ganz gemütlich ein Bier." in an other language than German, then you will get into trouble, because the idea that is connected with Gemütlichkeit is spread over many different words in other languages, and you have to find not only the best matching word for the actual context, but you also sometimes must provide the right context, that already is built into the original word.
The concept of quirkiness does not exist in German. (It exists in variations, but not in exactly the same way as in English.) Every German translation that is sufficiently close to the original meaning has a negative connotation. To be quirky means to be out of the norm, and being out of the norm is negatively connoted in most situations.
I don't really know the English word quirky, so I have no idea how positive or negative connoted it is, but the phrase "I like her ..." (in German: "Ich mag ihr(e(n)) ...") already provides a very positive context.
So, although all nouns in this list are negative connoted in other contexts, the whole sentence still is positive:
Ich mag ihre Verschrobenheit.
Ich mag ihre Schrulligkeit.
Ich mag ihre Ticks.
Ich mag ihre Marotten.
Ich mag ihre exzentrische Art.
- Verschrobenheit - eccentricity, crankiness
- Schrulligkeit - crankiness
- Ticks (plural of Tick) - quirks, spleens, tics, fads
- Marotten (plural of Marotte) - foibles, whims, fads, quirks
- exzentrische Art - eccentric nature, cranky nature, bizarre nature (also manner instead of nature)