And if it’s true, shall we skip the order by clause for Materialized View when executing create/refresh statement?
We tend to do precisely what the user writes into their query. If they don't want an order by they can remove it. I don't see any particular reason we should be second-guessing them here. And what makes the trade-off worse is the reasonable expectation that we'd provide a way to force an ordering of the inserts should the user actually want it after we defaulted to ignoring that part of their query.
But yes, you are correct that adding an order by to a materialized view is typically pointless. To the extent it is detrimental varies since even partially ordered results can save on processing time.