In the first chapter of Lucky Jim, Dixon thinks forward to his upcoming meeting with Margaret, wondering what she will wear. He decides that he can make himself compliment anything but her green Paisley dress and quasi-velvet shoes, which is, of course, what she is wearing that night. Margaret wears the dress again in Chapter 16 when she and Dixon officially break off their relationship. All of Margaret's clothing seems to be unattractive, but this dress is clearly something that Margaret likes a lot and thinks that Dixon will find attractive. The fake quality of the quasi-velvet shoes also seems to be specifically indicative of Margaret's lack of sophistication. Thus, the dress is symbolic of Margaret's unawareness when it comes to Dixon. The comedy of her wearing the one thing Dixon can't stand is also symbolic of the more general comedy of bad luck.
Mr. Welch's fishing hat and Bertrand's beret are symbolic of their pretentiousness. Mr. Welch fancies himself a man of traditional England, and therefore a man of the people, but the comedy of Professor Welch's hat lies in the implication that he has never fished in his life, or even met a fisherman, but still sees nothing amiss in wearing a fishing hat himself. Bertrand's social pretensions are more ambitious and continental, as signified by his beret. Dixon makes fun of Bertrand's beret specifically for its uselessness. It does not block rain or keep him warm, and is worn only for effect. Bertrand and Professor Welch are wearing each other's hats when Dixon meets them on the street in the final scene, and Dixon's comic enjoyment of this reversal and of the silliness of the hats more generally sums up his contemptuous feelings for the Welches throughout the novel.