He is made like that. That is his most notable feature about his appearance.
Brock's most notable feature about his appearance is that his eyes naturally appear closed. The rare times people see them open are when they are forced open unnaturally: from a hit with Charmander's Flamethrower in The Ninja Poké-Showdown, when being hit in the back of the head by Arbok's tail in Who Gets to Keep Togepi?, and when he makes a face for Larvitar in A Crowning Achievement. This has been the center of fan attention for a long time.
Here is a Picture of his eyes Forced Open. In ("Brock with his eyes open in A Crowning Achievement)
I love Weird things ¯(°_o)/¯