Orwell expressed concern about over-praise of mediocre works more than once, but I think that the essay you are remembering is ‘In Defence of the Novel’ (1936):
The trouble is that the novel is being shouted out of existence. Question any thinking person as to why he “never reads novels”, and you will usually find that, at bottom, it is because of the disgusting tripe that is written by the blurb-reviewers. There is no need to multiply examples. Here is just one specimen, from last week’s Sunday Times: “If you can read this book and not shriek with delight, your soul is dead.” That or something like it is now being written about every novel published, as you can see by studying the quotes on the blurbs. For anyone who takes the Sunday Times seriously, life must be one long struggle to catch up. Novels are being shot at you at the rate of fifteen a day, and every one of them an unforgettable masterpiece which you imperil your soul by missing. It must make it so difficult to choose a book at the library, and you must feel so guilty when you fail to shriek with delight. Actually, however, no one who matters is deceived by this kind of thing, and the contempt into which novel reviewing has fallen is extended to novels themselves. When all novels are thrust upon you as works of genius, it is quite natural to assume that all of them are tripe. To admit that you like novels is nowadays almost equivalent to admitting that you have a hankering after coconut ice or prefer Rupert Brooke to Gerard Manley Hopkins.
George Orwell (1936). ‘In Defence of the Novel’. In New English Weekly, 12 and 19 November, 1936. Reprinted in Sonia Orwell & Ian Angus, eds. (1968). The Collected Essays, Journalism, and Letters of George Orwell, volume I, p. 250. New York: Harcourt Brace Jovanovich.
In comments, WaterMolecule asked about the references in the last sentence quoted above. Coconut ice is a kind of boiled sweet made with desiccated coconut, condensed milk, and sugar. A liking for sweets is considered by some to be a childish habit that people ought to grow out of. Orwell alluded to this view in another of his essays:
At his worst, and also his most vital, in poems like ‘Gunga Din’ or ‘Danny Deever’, [Rudyard] Kipling is almost a shameful pleasure, like the taste for cheap sweets that some people secretly carry into middle life.
George Orwell (1942). ‘Rudyard Kipling’. In Horizon, February 1942. Reprinted in Sonia Orwell & Ian Angus, eds. (1968). The Collected Essays, Journalism, and Letters of George Orwell, volume II, p. 194. New York: Harcourt Brace Jovanovich.
The reference to Brooke and Hopkins seems to be a swipe at literary fashion and snobbery: Brooke wrote straightforward poems that were popular in the 1920s but fell out of fashion, while Hopkins wrote difficult poems that were neglected in his lifetime but by the 1930s were praised by the cognoscienti.