Maybe he has now done it, although I have been unable to find any reports of him doing so. But almost alone among world leaders, I think joined only by Kim Jong Un of North Korea, Mexicos's President known as "AMLO" did not basically immediately congratulate Joe Biden on his inauguration. Even very pro-Trump Brazilian President Bolsonaro did, expressing a hope to have good and beneficial relations between the US and Brazil, and although it was rather perfunctory and unenthusiastic, so did Russia's President Putin. But nothing out of AMLO, at least not right away.
Supposedly he somehow thinks that Biden is going to be less friendly than Trump. The one report I saw on this said that he views Trump as having taken a "hands off" approach with him, although I remember Trump making demands that he crack on migrants passing through Mexico from Central America with threats of tariffs at odds with treaty agreements. This is hands off? Anyway, somehow he supposedly thinks that Biden will interfere in Mexican affairs and try to tell him what to do, although I am completely unaware of a shred of evidence that Biden has said anything at all that would suggest that, although it is possible that he has and I simply have not been aware of it.
I posted here awhile ago about how it seemed that AMLO was favoring Trump, visiting him in the White House in September and making highly favorable comments. At the time I speculated that this might because he shares some authoritarian tendencies we have seen in Trump, with those only having become much clearer with his serious effort to overturn the US presidential election. Does AMLO's delay in congratulation Biden mean that AMLO approved of these actions by Trump? I do not know, but this adds to my concerns about AMLO. I was criticized by some commenters for making this point about AMLO, who is mostly a political progressive and certainly has carried out some progressive policies in Mexico. But I see nothing good for people in Mexico in having their president alienating the incoming US president for what appears to be no good reason whatsoever.