There's definitely a creeping sincerity entering the show now, but I don't think that's a problem.
The revelations about Mac's sexuality and his pathetic deference to Dennis are probably the most stark examples of the show undergoing another shift in how it approaches these characters, but I don't really have a problem with that. Mac has been psychologically and physically abused by Dennis since at least season 7 (in fact, Dennis has pretty much DENNIS'd Mac this entire time), so that element of the show jumping from a background joke to a central conflict feels right because it's not the sort of thing that can be ignored forever.
Not only that, but these people have all spent 14 seasons hating each other while continuing to hang out because they don't know any better - even Dennis failed to get away because he's addicted to the control he has over them. They've all done absolutely horrible things to each other, so after spending so much time in this horrendous dynamic there has to be room to analyse the negative effects. Classic Sunny episodes are often built around the innocent civilians whose lives are destroyed by the gang - it was only so long before the show started to turn that back on the gang and watch them eat themselves.
I suppose what I'm trying to say is that the gang have spent years torturing each other, and those years they spent torturing each other were always the elephant in the room. The only difference in the show now is that it's starting to acknowledge and analyse that elephant instead of simply allowing it to sit there and quietly affect things. One of Sunny's best attributes has been its ability to slightly tweak itself and update itself to stay fresh for so long. Essentially, it's grown up on several occasions, and I see them ramping up the sincerity and dramatic aspects of the show as another instance of them doing that.