Regardless if the story behind it, Gothitelle would stick out like a sore thumb in terms of being in Smash, even as an echo.
So far, Echo fighters have either been highly requested (Dark Samus, Chrom, Daisy), or were protagonists in their own series(RIchter).
I just don't see someone as ho-hum as Gothitelle getting in.
Hell, even Isabelle being a Villager Echo makes more sense due to her being a huge money-maker and being pushed into the forefront of Animal Crossing.
And then beyond ALL this, she would stick out super hard in the Pokemon roster as well.
Every Pokemon we have in Smash has either been marketed to hell and back in their prime, or was hugely popular.
? The mascot.
? Was popular way back when, still here due to seniority.
? Was a cheap clone, but at the same time, Pichu was marketed like crazy during Gen 2.
? I don't even need to explain this one.
? Was Gen 4's poster boy.
? Was a gamble, but became HUGE in Gen6 due to it.
? *Etika.wav*
Gardevoir I get, but Gothitelle would literally be a "We need to fill a quota" pick, nothing more. It's not a highly marketed Pokemon, was never popular, and doesn't really 'have' anything to its name to begin with.