1 - Mayor Dewey's version was already debunked as being excessively inaccurate. Doesn't mean he is wrong, but it significantly discredits his claim.



2 - Even assuming Mayor Dewey's claim is an accurate recount, unlike Pearl, it is still a 'story of passing'. Meaning Mayor Dewey's understanding of the events are either from direct word of mouth passed on through the Dewey generations(family story passed down) or from a recorded history book. In either case it all hinges on visual accounts from colonial era men who were desperately lost at sea. All of which, I highly doubt got out a measuring stick and climbed the Fusion to make sure their height guess was accurate.



TL: DR - Mayor Dewey's claims on the height are unreliable hearsay.





EDIT: Also after rewatching the episode:



3 - The mention of the Fusion being 50 feet was removed from Pearl's rewrite, suggesting the height was false.