I love TOS and as such I feel that they did a fine job, a marvelous job of stocking the episodes with fine and often underrated talent. Wracking my brains here. If you are looking for "terrible" then I have to confess I cannot think of it, at least in the classic sense. As the years have passed I have come to view TOS as fine art, from a different era albeit, but art nevertheless.
If there is one actor who gave me something notable to mention here, it doesn't qualify as "terrible", but damn, it sure is over the top and laughable, if not believable.
He was strangely effective if somewhat suspiciously exaggerated as the aging boy creature in Miri....an episode I love.
He played Chief Medical officer Carter on the USS Exeter, Captain Tracy's ship - another episode I absolutely love watching to this very day.
McCready's warning, facial expression, loud and laughable demise, his collapse from the Captain's chair, made me laugh with horror as a child and today I have to laugh at it for its comic book flare. Point taken, message delivered. The Enterprise landing party is in trouble. LOL!
If that performance wasn't proof enough then his next appearance underlined his colorful contributions to TOS forever as a sort of colorful and at-times understated secret weapon, as Barber in Spectre of the Gun - another of my all-time favorite episodes.
I have to wonder if McCready was sought out for his talent on other television shows more than any convenient availability, but having said that, while he may not come off as believable as a day-to-day sort of real life person, he fit perfectly in the TOS world of over the top characters and fantasy.
I mean no disrespect. In a way, he was so bad he was very very good.