Before anyone wants to be contrarian about this: no one is going to stop you from using a stranger's weeb digital art as your profile pic. It is not illegal to use someone else's art for your profile pic without permission and no one is saying it's should be.
The point of this post is that this is an extreme overreaction to a conversation that concludes with 'pwease ask this other person for permission, the art is of their character design not mine 🥺' in a niche art community that operates on an honor system for the sake of being polite to one another.
Depends on the purpose of the account in question. If it’s a personal account not being used for any financial gain, then it’s fair use. If it’s an account for commercial use, then it’s more likely to get struck as the owner of the image can claim their creation is being used for an economic benefit without just compensation.
Even if it's non-commercial a fair-use claim is far from guaranteed actually. there are four factors and it's just up to a judge to decide how to balance those factors, which basically means it could go either way in cases without a lot of precedent. A total crapshoot basically, unless you are using it not-for-profit, for educational use, and it has no effect on the wider market for the work. The factors are vague enough that lawyers can easily argue for and against fair use in many cases