The short answer is: No.
Each site would need to implement this feature themselves. Think of all the potential platforms out there: lemmy, reddit, pinterest, instagram, linkedin, tiktok, snapchat, tumblr, whatsapp, friendster, quora... this list is long. Like REAL long, and Lemmy is nowhere near the top in popularity. Probably not even in the top 50 yet. Why would site developers implement Lemmy sign on over any of these others?
We did have a concerted effort at some point to adopt openid which was specifically designed to unify all these signons in an open manner, but instead we ran into the Standards Problem: xkcd 927.