Sometimes, when a movie is universally panned, there’s a reason for it.
Skyline is another in a recent series of alien invasion stories that are told at the personal ground level, rather than with sweeping majesty like Independence Day was back when it re-opened the genre for public consumption. If you imagine War of the Worlds or Cloverfield, you are definitely on the right track. Aside from incidental plot and character arc divergences, these are all three (along with several others I could probably think of if I paused to do so) basically the same movie. So, what makes Skyline stand out? The first thing is the characters; every single one of these ranges between (at the high end) uninteresting and seedily unlikeable. The second thing is the plot, which, after establishing that the Aliens are Here! and People are in Danger!, effectively goes nowhere at all for the rest of the movie, up to and including a conclusion that I can only presume was meant to by heart-warmingly thought-provoking, or perhaps vice versa. The third thing is… well, look, if you need a third thing, it’s because you are more forgiving of badness in movies than I am, and I’m pretty sure I’ve never met the person that fits this description.
 I should say that David Zayas, who I hope you will recognize as Angel Batista on Dexter, really wanted his character to be likable, but the script simply wouldn’t allow it.