I looked at The Annotated Wind in the Willows - not annotated by Grahame himself, for the record - and found some interesting notes (pages 118-119):
- Garibaldi was actually quite well-known in Europe and beyond in the late 19th century, thanks in part to his frequent travels around the world. Playing a major role in unifying a formerly long-shattered peninsula is no small feat, and he received quite a lot of recognition for it.
- In 1864, Garibaldi visited Fowey, the town Grahame lived in (several decades later) when he cobbled together The Wind in the Willows, and it is likely that the Italian was well-remembered there. Small towns don't get visited by international heroes every day. He even shook hands with the future wife of Arthur Quiller-Couch, the writer who was supposedly the inspiration for Ratty.
Garibaldi merchandise was quite commonplace in Europe and elsewhere at the time, and it was not expensive. Mole is not well-off, as he tells Ratty before they go to his house:
‘I know it’s a—shabby, dingy little place,’ he sobbed forth at last, brokenly: ‘not like—your cosy quarters—or Toad’s beautiful hall—or Badger’s great house—but it was my own little home—and I was fond of it'
Therefore, small ornamental statues - relatively cheap, but not tacky - seem to fit Mole's persona very well.
- It is hypothesized that "heroes" is from a quote by Alfred Tennyson describing a meeting with Garibaldi. You may think it odd that Grahame should choose such an obscure reference, but keep in mind that Chapter 11 is titled ‘Like Summer Tempests Came His Tears', an allusion to a Tennyson poem.
For what it's worth, a contemporary biography of Garibaldi explains that he had great affection for the English people. He wrote,
'The English nation is by no means exempt from imperfections, yet the English are the only people who can be compared with the ancient Romans.'
Even before his fight for reunification in 1860 - which Britain helped, by the way, in part to keep France in check - Garibaldi's exploits in South America had become a hit among all classes. He also shared anti-Catholic sentiments, which of course permeated England, and was quite grateful for British help in Italy. In other words, Giuseppe Garibaldi was a big hit in Britain in the mid to late 19th century, and it's no stretch for him or his comrades to be in Mole's garden.