This is a bit random, but do publishers like Penguin move a lot of classics each year? Middlemarch’s rank in the Amazon USA list would seem to indicate it sells around ten copies a day. Does that seem reasonable? I thought it seemed a bit low, but my opinion is truly random. There are >12 different paperback editions, so that would seem to argue for popularity…
No, classics are not hot sellers. You will never see one on a bestseller list, unless Netflix has a new adaptation.
The classics do, however have very, very large total sales: they were a bestseller once, and they keep selling, a few every month, forever.