It’s an interesting one. I can see the issue with sloganeering in sports and while I don’t disagree with the general thrust of what DH is saying, on balance I think it’s best to not allow it.
On ‘Just Stop Oil’ specifically, I don’t think it’s a helpful phrase. As has been pointed out, it’s glib and idealistic, almost to the point of irrelevance. That said, I take it as short-hand for support for a wider movement to reduce use of/reliance on fossil fuels. But taken at face value it's not hard to see why it might land as pious nonsense.
More generally though, I struggle to see it as an overall negative. Those who think it’s bollocks, regard him as a hypocrite, etc. presumably aren’t going to change their behaviour for the worse on fossil fuels, so he’s lost some potential allies/supporters, but not much more. Environmental behaviours are contested and I’m sure he’s use to taking flak for his position, so I guess he just accepts this as a consequence of putting his opinion out there in this way.
Those who might follow his example (e.g. use public transport to get to events) and might go on to explore more about reducing their own carbon footprint… well, that’s a win for everyone. Obviously it’s a massively complicated problem to crack (way beyond simple slogans), but assuming everyone wants to see an overall global reduction in the use of fossil fuels and a move to more sustainable forms of energy in the future (whatever they might be), if he contributes something to those aims, however small, he’s done something good.