He looks and feels similar to Mario but plays drastically different.
Uair, Bair, Nair, Dthrow, Bthrow, Fthrow, Uthrow, first 2 hits of Jab, fireballs, Dsmash, Usmash, etc. are all very similar in animation. And if you were to describe any of those moves to someone who had never played Smash before, you would probably describe them the same way for Mario and Luigi. I.E. you would describe Dsmash as a break-dance move for both of them, and Uair as a backflip-kick or something similar.
If that constitutes as a clone, then yes, I'd say so. But they play so differently that they're not interchangeable.
Edit: Regardless, Luigi's persona in itself is kind of clone-ish. So it would make more sense for him to be similar to Mario than say Ganondorf to Falcon. They're brothers, and are basically attached at the hip. They rarely leave each other's side. So it'd make sense for them to have similar fighting styles.