The thing about Mickey Mouse is that while he’s not the first Disney character and not the first character to be created by Walt Disney himself (there were others before like Oswald the Lucky Rabbit and the Alice Comedies cast), he’s the first Disney character to come to mind. It wouldn’t matter if Donald Duck’s more popular than him in Germanic Europe (barring the British Isles).
What matters is that Mickey Mouse helped shape the Disney brand as it is today, especially when he was first conceived in the American depression. It’s not so much that he’s unique but that he’s instantly recognisable. Perfectly circular ears are undoubtedly easier to animate and much easier to recognise. It wouldn’t matter if he lead to similar characters like Minnie. At least they stand out more from relatively more realistic looking mice.
(One could make an argument for Felix the Cat being his grandfather and Oswald his father in terms of being black rubbery-limbed animals.)
They’re not any less iconic but that Mickey’s much more successful than they are especially in how and why he mattered to Disney.