Anyone notice how Dennis Hopper Copied Dean's Acting Style Later On?

You are thinking too hard. Hopper was a very independent type. His own man. Was his Babbaloogats (however it was spelled) in Cool Hand Luke mimicking something Dean did? Don't think so. I understand your point but Hopper acted like Hopper. Who else does?

Years later Hopper was still eagerly talking about bits of valuable acting advice he'd gotten from Dean.

... and Dean copied Brando, whom he idolized, though the admiration was not reciprocated.

By this time (1955), Brando was one of the top ten bankable stars and had been nominated for the Best Actor Oscar four years in a row: A Streetcar Named Desire (1951), Viva Zapata! (1952), Julius Caesar (1953), On the Waterfront (1954). He finally won for Waterfront.

Meanwhile, Dean's first movie, East of Eden, was only just released in April 1955; Rebel without a Cause was slated for release in October (a month after his death), and he had just wrapped up Giant, which would be released in October 1956, more than a year after his death.

Anyway, Dean attempted to call Brando and see him socially, but Brando rebuffed his attempts at bro-ship. “I gave him the name of a [psycho]-analyst, and he went," Brando wrote in his memoir, Songs My Mother Taught Me. "At least his work improved." Snark!

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I don't think he so much copied Dean as he was just employing the style they had both been taught, along with Brando, at The Actors Studio. I find all of the method actors if this era to show many similarities in their style. Actually to a nearly annoying degree.