As others have said - Legion, Doom Patrol, Challengers, and then later on Metamorpho and others really showed a development in style.

Many at the time were flummoxed by Marvel's popularity (and some still are!), but there definitely was a maturing of some of the storytelling at DC early in the 1960s, and by the end of the decade DC was telling more complicated stories with more complicated characters - not just in minor titles, but in the big ones.

Is it because of Marvel? I dunno. Sometimes SA Marvel just looks like page after page of people pummelling each other, followed by two issues of girlfriend angst.

DC developed and matured as expectations for the sophistication of entertainment increased and tastes matured.

But it was definitely a DC-styled maturing, rather than an aping of Marvel. Even bringing in Kirby and ushering in the BA brought about its own very peculiar set of books that are somehow VERY DC.