You can skip season 5, it's the definition of "tacked on".
The first half, maybe. But the second half is some of my favorite stuff from the whole show!
That said, Season 1 is pretty standard Season 1 stuff. Lots of random aliens-of-the-week, standalone stories, but it does help to set up a lot of stuff for future years. Just don't let its mediocrity discourage you from continuing.