Uncle Si is a Time Lord
That ubiquitous cup is his TARDIS, and his lunacy may very well be a reaction to the absolute horrors of the Last Great Time War. More realistically, Si has PTSD, and his goofiness is his way of dealing with it.
Si tells a lot of unnecessarily graphic stories about his time in Vietnam, often at strange times. They aren't all as made up as they imply. He has flashbacks and acts ridiculous and goofy to distract himself from them.