The live letter itself is very much an informal event. It always has been. I'm sure they're very serious and all business the rest of the time. The LL is a time to relax and loosen up. Yoshida-san is a fellow player talking to other players about what's coming up next. A little joke here and there doesn't seem out of bounds, considering the comments that likely show up in that ongoing flood.
They don't have to do all this. They could just have a letter and a slideshow and a very inhuman feel to the whole thing. Is that what you'd like to see? Maybe he should stop chatting with content makers when an expansion is coming up. I imagine the commentary on the forums would be even worse.