If a game is objectively good, then its performance in relation to any predecessor (whether in sales or reviews) is simply a matter of passing interest. A good game is a good game.
The only people who like to bash good games are emotionally deficient individuals who love to hate. And normally this behaviour is associated with an equally irrational animosity towards the platfom the game is on; and it's normally an exclusive title. So the question really is, is the bashing directed towards the game, or is the game being used as a proxy for having another bash at the platform?
That's not to say people who have played a "good" game but think it's a piece of crap aren't entitled to that opinion and can't strongly voice their opinion. But those people are bashing what to them is a bad game, and that's perfectly legit.
One sequel I think is going to underperform relative to its predecessor is Valkyria Chronicles II. America was by far VC's biggest market, yet it's pretty much the weakest market for PSP, but we'll never know since it's both a UMD and PSN game. But I'm not going to bash the game, neither am I going to bash the platform it's on. Rather I'm going to [figuratively] bash the people who made the decision to take it away from my chosen platform and put it on a platform that I don't enjoy using.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix