Just like you all, I am all for artists pronouncing the lyrics correctly no matter what the language is. Culturally, that is a great yard stick and artists should put in the extra work and care in that department. It is indeed a great experience when a song you know well is sung with great diction. But I personally cut a lot of slack given the practical difficulties artists face when singing songs that are in the tongues they are not comfortable with ( though I am not beyond making a remark or two about that! ). Another reason for that is a bit more personal,I am not very good when it comes to pronunciation in other languages and also tamil words that are of Sanskrit origin ( which I know is rshankar's soap box, I think I will perform miserably by his standards). So I am a bit hesitant to be too righteous about it.
But at a practical level, the problem is more acute when we know what the right pronunciation is. It definitely detracts from the enjoyment of the song. So it is a blessing in disguise and a boon that CM songs are in many languages that I do not know well enough to find faults in diction. So, in that sense, I tend to notice when artists murder Tamil more than artists murdering Telugu or Sanskrit, for example. Even the tamil brahmanical accent by Tamil artists when that is overdone and when not warranted is a bit distracting. I find it interesting the CM community in general seems to gloss over that. Some of artists tend to err on the side of too much sankritization of Tamil. Just to be on the safe side I suppose

rshankar, about your comment about the relative murder rate, I think it has more to do with frequency than anything inherent. There is really no reason to believe that any Telugu person to have less diction problems with Tamil than a Tamil person to have diction problems in Telugu. The problems are different kinds, possibly. But the frequency of Telugu mutilations are indeed very high given the high number of Tamil artists in the circuit ( the one that we are normally exposed to ) and the high number of Telugu compositions.
As observed in another thread, it is quite refreshing to hear an artist like Amrutha Venkatesh who seems to have the ability to sing in all CM languages with great diction.