Doomers can be a great voice of reason in a world that generally underestimate the climate danger.
But for me, there are two sharp dividing lines when it comes to doomerism:
One is when people use it to shut down others. The difference between “I need a break from thinking about this” vs “you need to give up! You need to stop trying!”. The latter is what’s not OK.
The other is when people exacerbate global warming. The difference between “I don’t know how to solve things” and “let’s put the pedal to the metal down the drain with air travel vacations and gas guzzlers and coal brokerage PACs”. The latter is what’s not OK.