I'm going to try to keep my comments to the original question, not because I think it's particularly interesting but because I suspect that the main issues ("Is Tolkien's work any good? Is its continuing influence on Fantasy harmful to the genre?") are going to be discussed again in this forum. Possibly more than once.
So, is it reasonable for Miéville to write a robust but fairly superficial criticism of The Lord of the Rings? Well, of course it is. If he finds a work reactionary and objectionable, he's entitled to say so: not as a matter of political free speech, but because engaged criticism is a vital part of a mature, healthy genre. That's so obvious that it's barely worth saying, but it is worth separating from the more contentious points, those that seem to have driven thedreameater to defend Tolkien from a perceived attack, which I'll come onto.
Miéville uses some provocative language, sweeping generalizations, caricature and unsubstantiated assertions in his piece for the Socialist Review. Is it reasonable for him to indulge in this sort of lazy criticism in public? I think Miéville knew exactly what he was doing, and I don't think it was lazy, for two reasons. One is that there's plenty of scholarly criticism of Tolkien out there already, and rehashing it in detail would be neither original nor interesting. For the purpose of Miéville's essay he merely needs to allude to its conclusions. The other is that for all his negative comments about Tolkien, his aim in the piece for Socialist Review isn't to prove that The Lord of the Rings is bad. Writing for a non-genre audience who are more than capable of spotting a feudal-patriarchal subtext for themselves, he actually seems to be mounting a roundabout defence of the books and the film of Fellowship, or at least those bits of them that he feels are still enjoyable. By not pulling any punches with the socialist criticisms at the start he keeps the reader's attention for the paragraphs where he describes what he values in Tolkien and post-Tolkien Fantasy.
That's not to say that Miéville's a fan. Which seems to be the third question wrapped into thedreameater's characterization of him as a "jerk": is it reasonable for the new guy on the scene to be so critical of the master who, intentionally or not, did so much to make the scene what it is today? Or should he show more respect to the great author who can no longer defend himself? Leaving aside the fact that it's traditional for the young turk to challenge the old orthodoxy (as Miéville says in the article, "Tolkien is the Big Oedipal Daddy"), it's important to remember that Miéville's comments don't come out of nowhere. The context for them is that for most non-genre readers, many dabblers and even a number of people who would regard themselves as fans, Fantasy is the shelves full of two-inch-thick "volumes" of Tolkienesque high fantasy. Miéville must constantly have to deal with people who assume that his writing is not just something that it's not, but something that he finds morally and politically offensive. So he tries to distance himself from it, which seems fair to me.
This post has gone on too long already, so the three word summary is: not a jerk.