Just now hitting season 2 of Supergirl, and the thing I've really started noticing is that that cast needs interesting guest stars to work.
The central cast of that show is fine. They're perfectly serviceable, but without something else to work with, they're in service to nothing. The highlight of the first season is when Barry Allen turns up, but it's not because Barry Allen is more interesting (even if he is); it's because they've got something fun to bounce off of.
The start of season 2 seems to suggest they knew that, because they go from two episodes with Superman (who might be the best Superman of the 21st century, but that's not a hard field to win in) straight to an episode with President Lynda Carter, and the show's so much more fun with that sort of catalyst instead of a mediocre monster of the week.