Yeah, Tekken, DOA, and Soul Calibur do have Button Mash friendly Characters. Eddie Gordo, Christie Whatsherface, Dunno the current BM DOA Character, haven't played any of the recent ones, and Maxi of Soul Calibur. If these characters didn't exist, Namco and Team Ninja would lose a lot of fans.
However, someone who takes time to learn combos, parries, and counters will always be able to beat a button masher. However, there are times button mashers do randomly enter in the ultimate move for the character they play, and can win that way.