IIRC Alph was considered to be a clone in Smash 3DS/Wii U and Rock Pikmin were also in consideration.
It's unknown whether Sakurai tried to implement this idea again, though.
Dark Pit and Lucina were also originally conceived as alternate costumes before being given gameplay differences.
Regardless, Alph being a Olimar costume makes sense since in Pikmin, it doesn't really matter all that much which character you're playing as since the gameplay revolves around the management of Pikmin. In fact, the Japanese name for Olimar/Alph is "Pikmin & Olimar/Alph", placing emphasis on the Pikmin moreso than Olimar or Alph.
Echo fighters in general are characters that don't have much to stand out. They're characters that work with another character's moveset, which is, once again, a gameplay design matter. Some movesets work with multiple characters, so these characters can easily be added in response to popularity.
Of course, all the animation differences, taunts and win poses help portray the character and make them more true to their source material, but are still just aesthetic differences.