Is Eventbus a Mediator or Observer Pattern?

The general usage of EventBus is to fire events. Use of the word Observer is better fit for that. Observer pattern uses events or messages to notify of a change to objects of interest about the object being observed(changed). Mediator also tries to de-couple the two implementations but is more concrete than Observer in the sense, it can know all about the two objects/interfaces and works as a glue to make those two work. Observer doesn't claim to know about the internals or even the interface. All it knows or cares about is when the event occurs, it needs to notify the objects who are interested.

Mediator could be a scenario specific setup whereas the Observer could be more generic.

EventBus being almost always a singleton within the scope of the application, I would definitely categorise EventBus as using Observer as its real intent in most cases is to facilitate messaging globally amongst the various modules/objects within your runtime.

Often, a given piece of code isn't intrinsically an example of one pattern or another. This is why they're called "patterns" (rather than, say, "implementation techniques"). A lot of software kinda looks like one pattern, but also resembles another -- this is good. It's best not to adhere to patterns for patterns' sake, but to use them as a shared vocabulary for discussing architecture.

EventBus is one such tool. I wrote it with Observer-like situations in mind, but if you structure your application appropriately it can play a Mediator-like role.