No they are not. It’s only considered a failure when the position is abused to prevent competition.
You can have a monopoly situation where competition just doesn’t bother to enter because the monopoly holder hasn’t abused the situation to make entering the market attractive.
Additionally, the government frequently grants monopolies because they don’t really want to see market competition due to the disruptive nature of the industry (e.g. utilities).
Monopolies are just a specific market condition that implies certain pricing dynamics and potential competitor strategies. They are not a failure in themselves and can frequently be a huge driver of competition. Nothing attracts a competitor like a stodgy monopoly with a thick margin and no penchant for innovation.