I think they could have done better than Zachary Smith.
In Britain, a few years before LiS, we had a series called Pathfinders in Space. This comprised three serials, shown in 1960 and 1961, involving visits to the Moon, Mars and Venus. The two latter featured a guy named Harcourt Brown, who in some respects anticipated Dr Smith. A UFO nutjob, he hijacks the fourth Moon rocket and takes it off round the Solar System. Yes I know, it's corny, and what passed for the special effects back then is excruciating to any modern viewer, but they made up for it by developing Brown's character as they went along. The Good Guys start off hating him, and with ample reason, but by the end he has become an almost sympathetic figure, and they part on quite good terms. Smith, by contrast, never rises above the comic-strip level, which is rather a shame.
Never seen Pathfinders. But Brown does sound like a good example of how to handle a Character like Doctor Smith. I never understood why the Robinson did not just exile him from the camp. After the Second or Third time, He almost get them Killed, he should have been thrown out.
Or if you are willing to do shades of grey. Have Smith be useful. He has skill that the Robinson need to stay alive. Have him make suggestion that are useful. At time let him come up with answer that save the day. Then as things start to get tougher, Have him make suggestion that don't agree with Professor Robinson view of the World.
At the end of the First season we have a problem that threaten the group. What if Doctor Smith comes up with a way to save the Robinson but at the cost of a group of Aliens lives?
Have Don agree with Smith and maybe Judy. Smith help them stay alive. He come up with Answer when Professor Robinson failed. Why should we disregard his answer now. He keep us alive in the Past.
This seem like a more interesting take on him than the "Oh the Pain, The Pain" take that was used in the OTL.