Orwell doesn't satirize imperialism, but he seems to say it doesn't work well. He acknowledges it's flaws, primarily the fact that the people aren't too happy.
At the start of the book, Mr. Jones is a successful farmer. His animals aren't being mistreated, though they are treated as animals and thus not equal to humans. They are mostly content (not really happy though), but they _jump_ at the chance to had a 'democratic' group (nation? farmyard? barnyard?) of animals.
So in Orwell's mind, imperialism may not be the best system, but it's certainly productive. However, it's vulnerable to revolutions such as the pig's revolution.
Also note that Orwell appears to be satirizing England/the US and Germany with Mr. Pilkington and Frederick, not really their political systems specifically.