I'd disagree. Having an easily identified image shown when a particular error is thrown can be really helpful when diagnosing problems, particularly when a customer / user is trying to communicate the problem with the site's support team. It's also quirky and friendly.
Unicorns are also supposed to be these rare, mythical beasts which I'd say is exactly the kind of imagery you want representing your server errors.
Unicorns are also supposed to be these rare, mythical beasts which I'd say is exactly the kind of imagery you want representing your server errors.