Again, it might just be one character. But he's been used to set the tone for both the teaser and initial trailer. I think it's ridiculous that some people are acting almost offended that some people might look at those and maybe make some assumptions of the tone of the game.
There's also been people who looked at the leaked poster and other things and thought that it looked like the game might be going for a somewhat more dark and gritty feeling this time around.
People will make assumptions based on what they are shown, that's just how it works.
That only works when you are selective about what you chose to focus on.
What have we seen so far?
-A city under siege in near ruins
-Tons of Guardians downed and under rubble
-A terrifying new-ish threat
-Loss of the tower and all those inside
-Heck we even get a glimpse of a new enemy type and dog-like creatures
But instead, some people have chosen latch on to a character that is quipping and decided to run with that for the tone.
There is quite a bit to dissect in the trailer regarding setting, hub worlds, social areas etc