I am absolutely in agreement with everyone who noted that the Party (especially Dustin) don't treat Steve great during S4, with the "Do you need to be told everything?" comments and such. So let me offer a slightly different explanation than them simply believing Steve is not cool/ is stupid.
Dustin is going through puberty. Yes, all of the boys are, the girls too, and what does one do during this hormonal time in their life? Rebel.
Now of course, all of the kids rebel to a certain degree, Max would have possibly entered the angsty phase even without going through all the crap with Billy and her mom, Eleven rebelled against Hopper during the summer of 1985, Mike has been a little shit for years (summer of 1985 solidified that), Lucas threw himself into sports to deal with the restless energy. Will is too anxious to allow himself to let go, but he withdraws into himself, working through things on his own. So what does make Dustin a special case?
Dustin doesn't have a father. But unlike Will who has Jonathan, Dustin's only proper authority is his mother and he'd never, ever hurt her by acting like an asshole towards her. She is anxious, he's the only important thing in her life, she says and Dustin believes her. He loves her deeply, but he's afraid to tip the fragile balance, she worries so much yet gives him all the freedom he wants. He could never repay her trust with being angry or moody.
Enter Steve. Steve isn't that much older than Dustin, but he's the only stable male influence Dustin has had so far. He often acts like the voice of reason, like an older brother that Dustin never had. And even when Eddie enters his life, it's different - Eddie is cool, but he hasn't been through all the crap with Dustin and Steve, he doesn't know what it's like. Dustin admires Eddie, looks up to him, but he doesn't have the deep level of trust with him that he has with Steve. So when the urge to rebel rears its ugly head, Steve is usually at the receiving end because he's the only one Dustin trusts enough to believe he'll handle it.
I'd like to imagine Dustin only becomes this bitchy when he's stressed. Maybe he snaps at Steve when he doesn't catch up as soon as Dustin thinks he should, but that doesn't mean he feels good about it. We see in S4 that Steve and Dustin have the sort of dynamic when one of them knows they overstepped and will apologize. And maybe it takes Dustin a bit longer to settle down and think about what things he says and how he says them, but does figure it out eventually.
After Steve's shift, when he's alone in his parents' house - no date today, Dustin always knows about Steve's pursuits - the phone rings and when he picks up, he hears Dustin's voice, clearing his throat. "Hi Steve. I just wanted to say that I thought about the talk we had today, about you not getting things, and it wasn't cool of me to say. You don't make fun of me when I don't get things in PE and so I shouldn't do this either. So - I'm sorry."
Steve smiles against the receiver and closes his eyes. Dustin's insults may sting in the moment, but he's always known he is a good kid, now a good young man. He's not a bully, not like Steve used to be. "Apology accepted, man. Thanks for saying that."
Dustin needs someone to rebel against and it's not fair to Steve that he's the one. But when he messes up, he makes sure to fix it, because Steve is not his friend out of obligation, they chose each other and that means everything. So yes, he will bitch that Steve doesn't carry a flashlight around like he does, but he will pack an extra one, just in case.
