To be fair, a lot of the mediocre 20th centruy stuff comes from the postwar period when urgent necessity was a rather good excuse.  lot of the harmonious old stuff was destroyed in the war, sadly, whole neighboroods or towns- worth of it.

An interesting comparison for architecture is between London and Paris, where the constraints on the appearance of the buildings are rather different (stringent height restrictions in Paris, plus "facadisme, i.e. the obligation to keep the outside as it was when refurbishing/tearing down a building). London has much more original architecture (from my uninformed point of view), but both cities have their share of ugly  and beautiful stuff (whether individual buildings or streets/blocks).

In the long run, we're all dead. John Maynard Keynes