I haven't seen a pipeline illustrated with a UML sequence diagram yet. I am not saying it is not possible (I think it is) but it would be a lot more complicated and abstract than your FBD which shows the actual architecture of the simplified processor.
The main problem I see with the sequence diagram is that all the stages happen in parallel and it could become really messy if you would try to illustrate things like forwarding or stalling in it.
Also with the FBD you can demonstrate how to build the processor. You can incrementally add instructions and see how the design evolves. You end up with quite a mess but if you followed from the beginning then it should be much easier to understand it.