I don't like to think that I'm someone who can mistakenly take seriously an intentionally hammy piece of acting.
It's possible that what seemed like exaggerated acting to him at the time fit perfectly, for the frantic, unbalanced, guilt-ridden Decker. Windom was the king of guest roles on TV at the time, but offhand I can't remember his being that intense in other shows. Maybe he was stepping into new territory he was unsure of.
We cannot be sure. However, if you ever see some of his other work, say, in The Twilight Zone, you'll know that Windom was not incapable of chewing some scenery. But to my mind it was always to good effect, as it was in "Doomsday".
I try to be fair and consider the times. Even in 1967 television was only, what, 15 years old? Most of these guys--including a certain balding Canadian actor of the time--were "classically trained". I think that means yelling loudly on an off-Broadway stage.
I'm not knocking them. That's what they knew to be "acting". The fact that television is a cool medium had not quite made it through the ranks yet. Some had figured it out, but most hadn't.
Regardless, I give Mr. Windom a huge nod in making that episode forever memorable.