Yep its all about opinions. Generally speaking when a team plays poorly most of the players tends to struggle. This could be said about 90 percent of all players. If not then the team wouldn't be playing poorly in the first place. Mata has his limitations, he made some good tackles yesterday but he will NEVER be a great tackler, that's just not his game. But his defensive frailties have been unfairly overplayed.
There will be games that won't suit him and he doesn't offer much defensively but as that article earlier posted shows his defensive contribution is no different to many other top class playmakers. But we rarely see them getting criticized for it. Also Mata was top 3 in terms of winning possession in the final third in 2012/13 season. This was BEFORE Mourinho came back to Chelsea and benched him. Funny how he's got a reputation of being lazy since then.
Not always - a lot of the really top class players can still look very dangerous even when the rest of the team is playing badly. Suarez, Aguero, Hazard, Ronaldo (when he played for us - although obviously still the case) - these are players who
always look dangerous (Suarez in more ways than one...). Not every player is like that of course, but it isnt black and white. Again for me, players like Mata and Kagawa are very much "the icing on the cake" - if you play well, they make you even better, but if you play badly, they will often be one of the worst.
As you said, he will never be a great tackler, it isnt his game - similarly Fellaini will never be spraying out 50 yard passes, and Carrick wont be scoring 10+ per season. This is fair - the thing that I think Mata lacks however, is some work rate/ethic as far as tracking back and defending goes. Hazard is a great example of this - when he went to Chelsea originally he was seen as quite lazy I think. Mourinho even came out and publicly said he needs to work harder. Now, he has very much added that to his game, as has Oscar (although he was already a fairly hard worker).
For me, work rate isnt a "not his game" quality. Some players like Zlatan can get away with it because they are a) world class and/or b) play in a less physically demanding position (striker). For a midfielder or winger though, its very hard to get away with it. Even Ronaldo was always praised due to his athleticism and work rate, and he was/is one of the players who could arguably get away with being lazy simply because of how good they are (in fact since joining Madrid I think Cristiano has gotten progressively lazier, but that is as much due to the circus that is Madrid, as anything else).
Work rate is important in the PL. Look at the comparative differences between Ozil and Sanchez in terms of how big a success they have been at Arsenal. Look at Willian, who was never deemed to be a *great* player but who is now pretty much undisputed first choice for Chelsea, and has been for some time. I already mentioned Oscar and Hazard (and the fact that Mourinho sold Mata himself). Look at how effective Suarez was at Liverpool, look at how Milner has become one of City's arguably most important players this season.
There is a definite theme and correlation here - that if you want to be a truly top player in the PL, you pretty much need to have work ethic, as well as skill. Naturally there are exceptions, but those are exceptions, for exceptional players where the sacrifice is worth making.
What I am basically saying from all this is that yes, Mata is playing well at the moment, and he deserves to stay in the team without a doubt. However for me until he improves his work rate in terms of tracking back and defending (or his "attacking" improves to "one of the best in the world" standard) he will never be recognised as a truly great player (not in the PL at least), and that someone like Di Maria you can argue would have a higher 'ceiling' for us in this respect.