PECS, what we think about him doesn't matter, what does matter is what the people he will be dealing with and negotiating with if he becomes PM think of him that matters and when they see his choice of outfit for this exceptionally large and significant occasion, they laugh at him, or worse still, snigger behind their hands. He is presenting himself to the big international beasts he will be dealing with then as a person, who can be completely ignored or eaten in one mouthful..
We can see already how Trump a much, much bigger man than Corbyn in every way, has made the USA a laughing stock since he became President. He's got away with it because the USA has a big army, with big guns and a lot of money.
Corbyn has the disadvantage that he isn't physically big, he dresses like a nonentity, and will have neither army, guns or money but everyone will be laughing at him, if they notice him at all.