to be good at

М, не совсем подходит, есть ещё варианты?

А можно сказать "He is in touch with mathematics/ he is out of touch with mathematics?

по-моему, это означает остал/оторвался от, потерял связь с

has no clue about, hasn't a clue about

в обратную сторону - https://idioms.thefreedictionary.com/be+clued+in

to know a thing or two about something

https://www.collinsdictionary.com/dictionary/english/a-thing-or-two

to know one’s way around something

https://www.merriam-webster.com/dictionary/know%20one%27s%20way%20around

А чем to be good at не угодило?

You don’t know a thing about fashion.

British people, like Americans, have a long list of phrases they use in this context.
◉ Americans say GOOD AT something, Brits say GOOD IN something 😉
◉ Brits use plural MATHS, where we use singular MATH.
◉ One way they express deep knowledge is with the words WHIZ or ACE if you know a lot about something: He's a maths whiz/ace. He's a whiz/ace in maths.
◉ If you're good at something because you've been doing it for a long time, then they say you're a PAST MASTER of it. (Americans don't use this expression at all).
◉ With fashion, it's not so much about knowledge as it is about instinct, taste, eye... so they say HAVE A FLAIR for fashion.
◉ On the more slangy side, if you're good at something in Britain, you may be considered THE DOG'S BOLLOCKS (literally the dog's testicles, so be careful using this one in mixed company).

добавлю свои 5 копеек для "не разбирается в чем-то"
1) to be ignorant in something. "she wasn't completely ignorant in fashion"
2) to be an amateur in something. "I'm not only an amateur in math generally but am especially weak in algebraic geometry" (начинающий, дословно: любитель, непрофессионал)

https://books.google.com/ngrams/graph?content=ignorant+of+fashion%2C+ignorant+in+fashion&year_start=1800&year_end=2019&corpus=26&smoothing=3&direct_url=t1%3B%2Cignorant%20of%20fashion%3B%2Cc0

https://books.google.com/ngrams/graph?content=amateur+in+math%2C+no+good+in+math&year_start=1800&year_end=2019&corpus=26&smoothing=3&direct_url=t1%3B%2Cno%20good%20in%20math%3B%2Cc0

ignorant of fashion значит более естественно звучит. Спасибо Uly!

по поводу amateur - ни в коем случае не настаивал, что это лучший вариант, но он имеет место быть, и в данном случае, чтобы вежливо сказать кому-то, что он не слишком эксперт в чем-то, может подойти лучше, чем просто "you are no good in <math>"

Hi Nick! AMATEUR is used for things that people do as a hobby or passing interest, so it sounds strange in relation to a knowledge or required school subject like math. That’s why there’s only 11 hits in Google: https://www.google.com/search?sxsrf=ALeKk01NzNIcFqad0DRBqqfkKc2Nwf386g%3A1611931157756&ei=FR4UYP63LYO1tAa_qbqACg&q=%22amateur+in+math%22&oq=%22amateur+in+math%22&gs_lcp=CgZwc3ktYWIQAzoECAAQRzoFCAAQkQI6BAguEEM6BQguELEDOgIILjoGCAAQBxAeOggIABCxAxCDAToCCAA6BQgAELEDUP3xJViBiiZglowmaABwA3gAgAGLAYgB5wySAQQyLjEzmAEAoAEBqgEHZ3dzLXdpesgBCMABAQ&sclient=psy-ab&ved=0ahUKEwi-lr3Ar8HuAhWDGs0KHb-UDqAQ4dUDCAw&uact=5 It just sounds weird.

ok I trust your opinion

👍🏼