This cartoon is about the judging of historical figures by modern standards of ethics and morality.
The cartoon was drawn during a campaign of statue toppling in 2020 that started with the toppling of a statue of Edward Colston in Bristol. Colston made money in the slave trade. His statue was erected to celebrate his later philanthropic donations.
The cartoon attempts to illustrate the problem of judging historical figures by the moral and ethical standards of today by showing the absurd (and hopefully comic) situation of a statue being toppled because the person represented by the statue was not a vegan. Veganism is generally speaking a very modern lifestyle choice that would be unknown to historical figures.
It is also about the phenomenon of people sometimes committing disproportionately excessive acts if they hold their views with a high degree of righteous zeal.
It is also about mission creep – the current campaigns about statue toppling began with racism but may extend to other areas.