Actually, it is described in the novel that Mr. Knightley has a negative reaction on FC going to London just for having his hair cut, although it isn't an outrage. From Ch. 25 (shortly following Emma's being disturbed and the desription of the neighbourhood's benevolent opinion on Frank in spite of his strange whim):
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There was one person among his new acquaintance in Surrey, not so leniently disposed. In general he was judged, throughout the parishes of Donwell and Highbury, with great candour; liberal allowances were made for the little excesses of such a handsome young man -- one who smiled so often and bowed so well; but there was one spirit among them not to be softened, from its power of censure, by bows or smiles -- Mr. Knightley. The circumstance was told him at Hartfield; for the moment, he was silent; but Emma heard him almost immediately afterwards say to himself, over a newspaper he held in his hand, "Hum! just the trifling, silly fellow I took him for." She had half a mind to resent; but an instant's observation convinced her that it was really said only to relieve his own feelings, and not meant to provoke; and therefore she let it pass.
By the way, if I am not mistaken, the haircut was only an excuse for Frank to go to London and buy the pianoforte for Jane. It is rather a silly excuse (I'm not sure whether he perceived the danger of being judged for frivolity by a conservative country society) and buying the pianoforte is a show of his reckless attachment to Jane, but he did it to give her something she would be happy for, however badly he executed it.
