RE: Mary Hartman x2 - if you remember, it was intended to be a take-off on soap operas, so the stilted acting was probably meant to satire the soaps. At least until it devolved into more of a straight-up comedy.
In the 90s, my kids loved Blossom. Aside from Joey Lawrence's over-acting...a few years in, the father in the show got involved with a British girlfriend, played by the lovely Finola Hughes who is really British. But she had a daughter on the show - maybe 6 years old, who was obviously not British, and had the single worst English accent in the history of bad acting. It was probably too much to expect such a young actor to do an accent. After a number of shows, they gave up, and the kid started talking with her natural American accent.
Not to pick on kids - but the runner up had to be the young actor in Small Wonder. What idiot decided that robots had to speak in a stilted monotone? They can design a robot to seem totally human, but they can't get her to speak normally...even in the 80s? A truly awful program.