It's not in itself something a C programmer would ever really find themself doing, but as an algorithmic question it's fine - it's testing knowledge of looping, radices, strings etc. In C, you normally have to output the string in reverse and then reverse it, so you need to be aware of that. It's also a problem that's quite easy to explain (programmers are familiar with numbers and strings), but due to it's library availability, most people won't have actually written it themselves so it's not just a case of having done it before.

Coincidentally, I was asked this exact question in a phone interview this week. I didn't have to give exact code, I just walked through the principal of it (loop for each digit, dividing by the radix each time using the leftover specify the character (can be a lookup in an array), possible to use bit shifting in base 2/8/16 situations).