The thing is that, in general, the acting in the shows you mentioned worked for the show. Star Trek's acting was campy and melodramatic, but so was the writing. No one thought, hey, let's get Kevin Sorbo for his acting chops in Andromeda, but they wrote in a way that his bad acting didn't matter as much (it still wasn't a great show though). And I wouldn't disagree that TNG seasons 1 and 2 were not great, but they had enough decent acting for it to not be totally immersion breaking. Riker, Tasha Yar, and Wesley Crusher all were intolerable.
On the other hand, the acting was wooden on Farscape, but the show tried hard to be serious, at least in the first season.