I love both shows (but I love South Park way more), and while the critique is definitely valid, it’s also unfairly trying to hold Family Guy to a standard that it doesn’t hold itself to.

Family Guy knows what it is, and it doesn’t try to be more than that. It knows that the plots are stupid and that the cutaway gags are lazy, but it also knows that the formula works and people enjoy it. Not everything has to have biting social commentary and subtle, intelligent humor. Sometimes a show is just an alcoholic moron getting himself and everyone around him into insane predicaments, split up with nonsensical one-off cutaway gags that do nothing to contribute to the plot.

If Family Guy pretended to be a higher-minded show than it is, the criticism would be more justified, but the people who make it know what it is. Criticizing Family Guy for being random and stupid is like criticizing Robot Chicken for being random and stupid. Both shows are meant to be random and stupid.