Regarding Larry Merchant, I'm a fan of him in general, and he's always good for a couple laughs during a broadcast, but there's no denying that his performance on the air has slipped dramatically the last few years. Watch an old HBO telecast from the 90's, and you'll be amazed by how much insight he provides and how much talking he does. Nowadays, he basically sits there in silence for 3 minutes and then chimes in with a one-liner in between rounds. Steward is really the color commentator now with Merchant being the third guy.
As for getting negative and cranky, I think his recent comments on Mayweather are a pretty good indicator. Back in the 90's, he would constantly defend Pernell Whitaker for fights that were labeled "boring" and that weren't crowd pleasing. There were tons of fights where Whitaker would outclass his opponent, but the crowd would be booing over the lack of action, and Merchant would call out the fans for being foolish to boo Whitaker. Now Mayweather turns in a performance that's similar to many of Pernell's, and Merchant complains that Mayweather is boring, it's hypocritical.