I don't know about Rivers' history with Johnny Carson, nor did I see any of her comedy routines. I only knew her from her red carpet commentary.. I wasn't a fan of Joan Rivers. I didn't like her, nor did I dislike her; IOW, I didn't care for her one way or the other.
That said, she had her own, unique style. When she opened her mouth, you KNEW it was Joan Rivers! Even without seeing her, you'd know her when you heard her. In that sense, she was better and more talented than the cookie cutter actresses we have today.
Finally, I like her comment about Barack & Michelle Obama. Rivers said that Barack was gay, and that Michelle was a tranny! She said it unapologetically and without equivocation. You can find the clip on YouTube.
Those are my thoughts on Joan Rivers.