Dee's brownface scene is designed to be cringe though. Your not supposed to be laughing with Dee, your supposed to be horrified that she thinks this impression is funny. And then laughing at her impression failing and leading to her getting embarrassed. Even Frank comments on how fucked up the brownface is. It's like the scenes in The Office when Michael is being terrible and for the most part Michael is never portrayed to be in the right either. I can understand why people would find it offensive, but it's making fun of how terrible some white people are to me.
I think the quality sustains until Season 11. 2-7 is the clear peak, but it's still a great show in 8-10. The last four seasons are inconsistent, but it still hits most of what I find funny and I'm usually laughing throughout. They still put on at least 2 or 3 absolute classic episode a season and to me that's great for a show over a decade old. I think the reason I love Sunny is that there's so much physical comedy and it seems rare for a show to have so much these days. I prefer the new Sunny seasons to the last 3 or so seasons of Curb, but I might be in the minority there.