How will other countries take us serious when Biden can't even walk up a flight of stairs?

The whole world is laughing at Biden.  

People trip dude.  I'm sure you have,  so  you should "be ashamed"!? What now?

FDR was in a wheelchair, King George VI had a stutter, and yet everyone seemed to take them seriously.

The entire body of the United Nations laughed at Trump right to his face and then life went on.

That's probably not really a problem. Those sorts of things can happen to anyone. Moreover, national leaders generally put less stock in a simple faux pas like that than they do in someone's actual intelligence and their policies. The reality is that Donald Trump was in extreme laughing stock on the world stage. World leaders often literally laughed at him. There's probably no world leader out there who respected Donald Trump. Some of them feared him, but in the same way that you might fear a young child trying to drive a car. You're worried about the damage that they may cause but you don't actually respect them.

Things like that happen.  Gerald Ford fell down some stairs, George H. W. Bush vomited on the Japanese Prime Minister, and Jimmy Carter was attacked by a rabbit.  

They didn't have a problem when Trump stumbled walking up a ramp. That's just a tilted floor.

Worse is that he mumbles and has to read everything.

It is not Biden who is turning the Country into a laughing stock.
The fools who voted for him are responsible.