I watched LIS when I was kid and at the time I thought it was fine. When I discovered TOS it was like a light had been turned on.
I have first season LIS on DVD. It comes off better in b&w and the first season isn't so goofy. It can still be enjoyed on its own terms, but it's nothing like Star Trek.
Apparently Dorothy Fontana really disliked LIS.
All the Irwin Allen sci-fis are like that: decent concepts and reasonable beginnings that eventually devolve into goofiness.
wasn'r wholly immune to the lighter touch. If you watch TOS' second season you can see an overall lighter touch particularly with episodes like "The Trouble With Tribbles," "A Piece Of The Action" and "I, Mudd." But third season, despite being panned, makes an effort to get away from that.