i think you should be able to say what you truthfully think about any performance.... i reckon it is perfectly acceptable to say you didn't like something or that you think someone played or sang something poorly, that is, after all, the point of an opinion....as for attempting to "correct" a performance (as mentioned by robert) well, perhaps thats the no-go area as everyone who plays has an interpretation, and, well, maybe they wanted that wrong note there on purpose....its like people in the classical field who make a great fuss (a negative one) over a player who may feel a piece needs a few more notes here an there, or slightly different timing or passion...no-one but the composer or the player truly knows what they are trying to convey in both technique and sound...so all we are left with is our "opinion" as to whether or not we liked it, or if it was different to our knowledge of the original performance/composition....there is way to much dilly-dallying with opinions in this world...if more truth were told i believe the world would not be in the state it's in...by conveying the truth a person knows exactly where they, and you, stand, and if you speak your truth's quietly and without rancour, i believe people can deal with that, far better than any well-intentioned lie......you don't need to "suck up" to someone before stating your opinion..that's diplomacy and look where thats got the world!!! just say what you think, but in a non- aggressive/personal way...there's no need , imo, to "soften" the blow....anyway, just my contribution to this topic fwiw...
dennis