Asking about company dress code is a perfectly good question, and it's one part of understanding the company's overall culture. Any sane recruiter or hiring manager wants you to understand and be attracted by the company culture, and will be happy to answer questions about that sort of thing. Furthermore, they may give you brownie points for asking since it shows you are interested in making a good impression and in understanding the nature of your job on the whole.

I had this fear too. It's a common fear, I think! It probably made a lot of sense until the last few decades of recent history.

But now that we have CEOs wearing everything from suits to sweatshirts, there's no way you could possibly be expected to magically know what a company's dress code is ahead of time. :)

Asking is completely sensible. Ask either the person you did a phone screen with or the hiring manager.

This is a good question. When setting up an interview, don't be afraid to ask a question. Especially when you are speaking to a recruiter.