I don't think these movie are crap, by any means.
But I think the serious became, less than stellar, starting with Son of Godzilla. Great setting for a story with the weather alternation device and mutated insects and...etc. But the dopey looking Godzilla and father and son bonding was not my cup of tea.
Then Destroy All Monsters was alot of huffin and puffin but didn't blow the house down.
Godzilla's revenge, child's variety hour, with mixed messages for kids.
Godzilla vs Hedorah, not to shabby but failed to please the masses.
Godzilla vs Gigan, had a good heart but not the production values to match (I love it thou).
Godzilla vs Megalon, wacking but fun.
And then the Mechagodzilla's are good fun but it always sat uneasy with me to watch Godzilla get his ass hand to him so much (Terror Of Mechagodzilla). I always thought thats something thats more Gamera-esk.
But to answer: "When did the Showa Age go Wrong?"= Son of Godzilla
The dead animals in KOM are a result of high powered signals sent from aliens,to awaken the dormant monsters. Animals with similar senses are overloaded from the signal &drop dead. That way much of the world will be destroyed in prep for the invasion.