Thanks to Usual John for pointing out this Blondie episode with a mystery. He asks “Is this a reference to someone in Blondie’s flapper past?”
We noticed later that Josh at The Comics Curmudgeon also features this cartoon. But he doesn’t truly answer the mystery, so this independent find doesn’t obviate the usual spirited CIDU discussion.
Comics I Don't Understand 
Yesterday was Charlie Chaplin’s birthday, might that have been it?
It was in their attic and forgotten, so who knows? In any event, the early history of the strip is at https://www.loc.gov/loc/lcib/0006/blondie1.html
With all due respect to the extended history of this geriatric comic strip, there is just no way that the current writer(s) would make a concrete reference to any obscure character from that far back.† The bowler is merely a random artefact, and the humor resides exclusively in Blondie’s excessive reaction to an otherwise innocuous object.
P.S. The fatal flaw in the artwork of this strip was inflicted by the colorist. While using red for that bowler does make it look “distinctive”, it no longer seems like an authentic, old bowler (which would have been brown or black). Instead, it looks more like a (modern) plastic children’s toy.
P.P.S. † – Even if we assume that the current writers are that aware of the strip’s history (which I doubt), the syndicate is definitely aware that only a vanishingly small percentage of the strip’s current readership could be expected to have any knowledge of such ancient details. It makes no sense to write a mass-market strip for 0,05% of the population.
But how did the derby get into the trunk in the attic in the first place? Did that “someone” leave it? Was that “someone” Dagwood himself? When he says “I knew it”, he knew what? That the hat was there?
If it reminded her of someone famous, in addition to Charlie Chaplin there’s Stan Laurel, Oliver Hardy, and Spooky The Tough Little Ghost, not to mention Winston Churchill and hundreds of other people in England.
Generally the Sunday strips are colored by the cartoonist, or in this case someone from the team is likely.
I think the strip’s origin years — Blondie as a ditsy flapper, Dagwood as a millionaire’s son disinherited for marrying her — were effectively erased once they became an archetypal middle-class couple. There are occasional gags about unseen Bumstead forebears when Dagwood shows the kids a photo album, but if anything the strip has pointedly denied Blondie and Dagwood any in-strip history. Come to think of it, does anybody remember the last time a Bumstead relative turned up?
Alexander and Cookie progressed to adolescence, perhaps to stay in sync with the long-running movie series, but they too have settled into legacy strip eternal youth.