A nickname is only funny if the person wearing it laughs too.

The test is not whether the group laughs, it is whether the person does, and not the polite version. Nicknames that stick without consent follow people for years and get used by people who never even knew the origin. Most of the time nobody means harm, which is exactly why nobody checks.

Ask once, privately, whether they actually like it, and believe the hesitation if you hear one. If it needs to stop, stop using it yourself first and the room follows faster than you expect.