I’m not sure I agree with either of your points:
(1) I don’t think that Fellowes is defending the class system at all. Or if he is, it’s in a “hey, you might think that every noble was an utterly selfish fascistic idiotic leech who basically treated everyone else as a serf, and it wasn’t that bad” way, as opposed to a “this was a good system that we should have today” way. The system DID exist, it DID break down, and watching that breakdown is fascinating, which is part of what makes the show good. If you have read a quote from Fellowes in which he expresses actually longing to return to the class system of the past, please point it out.
(2) No one is (as far as I can tell) attempting to trammel your right to express any opinion you like, certainly including your right to call the Earl a jerk
(3) That said, I think that saying he’s a jerk is a bit facile and reductive. He’s someone who was born into and grew up in a system that is incredibly alien to us. It’s easy for us to say that he didn’t deserve the wealth and power and influence that he had (and, quite clearly, that’s true). But remember that he didn’t invent the system. He didn’t wake up one morning and say “hey, I think I should be rich and powerful and live in luxury, just because”. He was born in the system, a system which had existed and done fairly well for centuries. Blaming him for the inequities of the system, and his inability to singlehandedly cure them, is a bit silly. Instead we should judge him within the role he plays. How does he compare to other Earls of the day? Well, we don’t actually see many of them, but I’d say that if we take his good qualities (and I think it’s pretty clear that, for instance, he takes his responsibilities as a noble very seriously, and it would in fact trouble him greatly if there were a plague or famine in his village, and he would do his best to alleviate the situation) and bad qualities (terribly mismanaged his wife’s money, generally mismanages the estate, frequently hidebound and stubborn) and balance them against each other; well, he’s neither a saint nor a devil. And I think that on the whole, I’d put him on the good side.
Let’s put it this way: for some magically hypothetical reason, you are going to be transported back in time to 1910 and will have to live and work as a farmer in England. You have two choices:
(1) you can live on a farm in a village where Lord Grantham is the earl
or
(2) You can live on a farm in a village where a totally random person is the earl
Which would you choose?
Certainly, if my two choices were to be a servant or underling for the Earl vs. a servant or underling for that newspaper guy I’d pick the Earl in a heartbeat even though the newspaper guy, being a self-made man, is more admirable by many modern standards. (Granted, the newspaper guy is a bit less likely to go bankrupt and leave me unemployed…)