I think the writing is a bigger problem than the cast. When it's good, the cast has no problems being funny. I just can't understand how something like the Iceland skit made it to air unless they just ran out of time and had nothing else to fill airtime with.
I think Keenan Thompson, Aidy Bryant, Beck Bennett, and Heidi Gardner are all serviceable cast members that can be put in a sketch and be funny so long as it's a decent sketch. Colin Jost and Michael Che are very good at Weekend Update, which is still, as it always was, a highlight most weeks.
Kyle Mooney and Melissa Villasenior are both funny and very underutilized most of the time. Pete Davidson and Kate McKinnon are both overused, IMO, and aren't as funny as their airtime suggests. I guess it's popular since it's a reoccurring bit, but I don't really find the "Chad" sketches funny at all.
They do need new blood in the cast though to shake things up.