I'm slightly confused - this is the only portrait of Pushkin I can find by Tropinin, but it doesn't show him below the waist; maybe you mean [this 1827 portrait of Pushkin by Orest Kiprensky](https://commons.wikimedia.org/wiki/File:Alexander_Pushkin_(Orest_Kiprensky,_1827).JPG)? It also doesn't show him below the waist, but does include a swath of tartan over his shoulder.

Anyway, Scotland was very fashionable in the early nineteenth century - or rather, a certain romanticized view of Scotland (which is still seen today). This is largely seen as originating with Sir Walter Scott (1771-1832), whose interest in history, poetry, and fiction led to him publishing first several collections of poetry and long-form works in verse, like the Lay of the Last Minstrel (1805), Marmion (1808), and The Lady of the Lake (1810), all about sixteenth-century Scottish feuds, politics, and romances. These were followed up with novels on similar themes that immediately made him a literary success - some of which are still well-known, like Rob Roy (1817) and Ivanhoe (1820), and many of which would now be considered obscure despite their popularity in their day.

In 1822, George IV made a state visit up to Edinburgh, where he was met by a crowd of onlookers, some of whom were noted as being in folk dress, which helped bring the Scottish aesthetic to an even wider audience - but the myth of the specific tartan assigned to specific clans, and the tartan mystique in general, was already well established. Even as early as 1805, periodicals were noting that fashion was embracing the pattern; in Colombo, Sri Lanka, at the St Andrew's Day ball in 1808, all of the women attended in tartan. Historicism in general was in fashion, and you can find numerous references to Gothic/medieval and Renaissance clothing in extant garments, magazines, and portraits - tartan was strongly associated not just with Scotland, but the Scottish past because of this romanticism.

It wouldn't be strange for anyone to be depicted with tartan accessories in the 1820s, but it's particularly appropriate for Pushkin, despite his geographic and cultural circumstances. Like Scott, he wrote in the romantic strain, focusing on love stories, wild adventure, magic, and folklore, mixed with a healthy dose of nationalism; his first work, Ruslan and Ludmila (1820), was an epic poem about a chivalrous love story set well in his country's past, and his second, The Prisoner of the Caucasus (1822), had a more contemporary setting but still focused on adventure and the "exotic" Caucasus region. It's highly likely that Pushkin would admire the world of Scott's fiction and deliberately ally himself with it in paint.