Asklemmy

Context matters a lot:
-Discussing a general topic at length among peers, and someone says "Dude, no one cares"? They're telling you to stop taking, and a) are annoyed, or b) in a mood to put you down. Use the rest of the context to determine your next move (e.g. stop discussing it, point out why it's important, or leave).

-Discussing an insecurity of yours, and someone says "Dude, no one cares". Usually means they think the basis for your feeling insecure is unwarranted, usually though not always followed by a more direct statement on said insecurity. Meant to be reassuring, as someone else said.

-In some cases, "no one cares" means that in the speaker's experience, the amount of people who do care about thing x is marginal, to the point that paying too much attention to that warps the understanding of the situation. This is tricky - by way of example, I've said "no one cares if people are trans or not" before (I've learned - this is a deliberate example, stay with me). Taken at face value, this is blatantly untrue - some people care a lot, both in a negative and positive sense. But as just a guy in the world, this is truly my experience - other people being trans generally isn't something the majority of people care about. I can think of only one person I've met who does care (negative sense), and he's generally a weird guy anyway.

As you can imagine, though, going around saying this carries some danger, as it can gloss over the risks posed to trans people by those ostensibly marginal figures. My saying no one cares is a product of my necessarily limited exposure to and experience of the world. The best way to approach this IMO is considering, and speaking respectfully to, the speaker's blindspots - whether or not the people who care are truly marginal demographically, the impact the people at the margins actually have in the topic of discussion, etc. Depending on the exact topic, either it will be demonstrated that it is essentially true - while it's doubtless someone cares, the number of people and impact they have on the topic is marginal to the point that this is irrelevant to the topic at hand - or identify a blindspot that hampers the speaker's understanding of the situation.

I will note that in speaking of context, you may not be neurotypical (took a couple tests at some insistance by my kid, and despite being an odd duck in general, odds are I am) - unfortunately I can't speak to how to elicit and identify full context in that case, but others here might. Apologies if that's the case, where "use the rest of the context" probably sounds like "draw the rest of the fucking owl" in an owl drawing tutorial.