Years ago I lived in walking distance of a yearly Highland Games.
The day was usually hot and the grounds sunlit. Headgear for shade and in something other than wool was a good idea.
There was usually at least one Pipes Band or Pipe Major who wore tall pith helmets and looked good -- but that was a band uniform.
I recall perhaps two or three gentleman attendees who wore the safari style helmet.
To my eyes and in that venue the helmets looked good.
No more of a costume (in the US) than bonnets or Aussie/safari style slouch hats.
And more attractive than bucket hats.
But my own preference is for cotton caps or brimmed hats.