There is a good argument to be made that the attendees could at least have been seen to be doing as much as they can to minimise their carbon footprint while attending an event largely about carbon emissions, rather than being seen to be doing as little as they can.
Look at the US delegation. It has taken the most wasteful approach it can possibly take by the looks of it, by bringing Air Force One (55ish people on a 747 is a hell of a CO2-per-person cost), but then also flying over Marine One inside a C17, along with a couple of Black Hawks and around 80 cars... and because they've flown into Edinburgh instead of Glasgow, they've then driven up and down the 30 miles of the M8 in convoy every day with the Black Hawks overhead.
Obviously, the President of the USA needs his security, but really... 80 cars, three helicopters and two of the biggest aircraft in the world, plus 60 miles each way for most of them every day just... looks bad (and you shouldn't be doing things that make you look bad). Land at Prestwick (seriously, it's literally built for it; US military and security services routinely use Prestwick and have done for decades) and take the Beast and ten cars maybe?
The USA is hardly alone on this, it's just the most obvious and probably the most egregious example. There's over 400 private jets in Scotland at the moment for COP26 delegates, and - aside from your own personal, blue-white-and-gold 747 - that's about the most carbon intensive way to move about the planet. Our own PM has taken a "private" (charter) Airbus A321, despite the train line between London and Glasgow being one of the best in the country...
Just taking average figures, COP26 will result in the same CO2 emissions for the ten days its on as if the population of the city of Glasgow suddenly increased by 30% - roughly the annual CO2 emissions of 4,500 average UK citizens, or about 165,000 people for ten days - with about 80% of that being the flights to Scotland.
Yes, COP26 will hopefully result in a reduction of carbon emissions not only many times over but orders of magnitude more than it will cost, but when the attendees are not being seen to be mindful of their own carbon emissions in getting to it, the result is public cynicism.
You don't have to look far to see people saying "What's the point in me [insert carbon cutting activity here] if Joe Biden's going to emit more carbon in two weeks than I do in a year?".