Many, many TV stars wore hairpieces. Especially in those days. A balding dome ages a man. Shatner, when he had a really good hairpiece, looked amazing. If I could have afforded a great piece, I would have gotten one as well. Instead, I shaved my head - which is very common now, so it doesn't stand out (Yul and Telly were right - women DO love it). I get that Shatner's thicker, fuller and wavier 'do in the 80's did get laughable, but to give him shit in the 60's is ridiulous. Does anyone chuckle over DeForest Kelley's eyebrows? Anyone ever track those? Compare his Corbomite Maneuver style to The Devil in the Dark. In the latter, they are so full and upswept, you know there's something going on. I'm also not convinced he's not wearing a rug either, the way it always laid (I'm pretty certain was in the films). Walter Koenig had a wig in his first few episodes that looked worse than anything Shatner wore in any Trek production. Just about every woman in the crew wore huge wigs.
Taking it away from Star Trek: Marty Landau wore a piece for Space:1999. Larry Hagman - all through Dallas. And it changed every year. Terry Becker on Voyage to the Bottom of the Sea wore a toupee in every episode. Jonathan Harris had hair transplants during the course of Lost in Space. Almost the entire cast of UFO wore hairpieces, including Ed Bishop and Michael Billington. Hair was part of the image. It still is, but you can get away with more now.
So, TOS era Shatner: cut the guy a break, he was far from the only lead wearing a rug and the wigs he had were generally very flattering. 1970's through Movie era Shatner - have at it, at least until Star Trek 6. He started looking much more natural then.