No, that's passive-aggressive. You can be assertive without behaving like a redneck with a power mullet. Diplomats, ambassadors, salespeople, PR people, advertisers, other players do it every day.
Yes, you can be an asshole and say something like, "Hey Shitface! Take that thumb out of your ass and get the fuck over here!", or you can say something like, "Say, Captain BlasterDude, can you give me a hand with these guys? They're resistant to my main damage type and I think they'll go down more quickly with your assistance." or "UltraMegaOKBrute, we're having some trouble keeping these spawns bunched up, do you think you're up to helping us corral them?".
Teaming is a relationship, and like any relationship, if you don't communicate, it's a bad relationship. They can't read your mind any more than you can read theirs, so speak up.