"is bad at" is a correct and usable part of a sentence in written English.
You can use it whenever you want to describe someone's capabilities in a certain activity. For example: "He is bad at understanding math."
The UK is bad at asset transfer.
But it is bad at saying no to flashy projects.
When he is bad, at least, he is accountable.
The UN is bad at fast-moving crises and standing up to strongmen.
"These guys exist because Microsoft is bad at mobile e-mail," says Mr Modoff.
He has twice lost elections: his party is bad at ejecting losing leaders.
He says he is "bad at" remembering not to overexcite the children before bedtime.
The problem, he says, is that the left is bad at making arguments.
She is bad at technical thinking, but good at expressive language.
(This is something that software is bad at, but humans find easy).Crowdsourcing has also spread into some unexpected areas.
Worse, the agency has become a bureaucratic behemoth that is bad at helping job-seekers in today's market.
