For me, there are two kind of sharp dividing lines when it comes to doomerism (and people do one or the other or both, they’re independent).
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.