Clayface was a major villain in the Batman titles of the early sixties and, in fact, even appears in the opening credits of the '66 TV series, although the character was deemed too scary (or more likely too complicated as far as special effects)to actually appear live. I never understood, though. If he makes himself resemble a bird, he gets the power of flight. If he makes himself resemble a bear, he gets the bear's strength. So I guess it makes sense if he makes himself resemble Superman, somehow duplicating him exactly right down to the last bit of Kryptonian DNA, he would get Superman's powers. The part I don't understand is…why doesn't he just ALWAYS make himself Superman?
The Doom Patrol get their own mag. The similar X-Men debuted from marvel at the same time the DP showed up in MY GREATEST ADVENTURE and there have long been charges of idea thievery. DP creator Arnold Drake would, ironically, write X-Men just a few years down the line.