Luigi wouldn't have been an Echo, no. He wasn't a costume to begin with either. But more importantly, he was heavily different immediately into Melee.
The only two who became Echoes were Dark Pit and Lucina and only because of their debut timing did it work out. Dr. Mario isn't just "miraculously" still a clone. He started off similar to Mario but got massively changed in 4 to the point they played widely differently. Luigi still plays quite different from Mario even in Smash 64. They shared animations, but were still quite different overall.
Simply being based upon another character was never what made someone an Echo to begin with. The first and most important thing is the same bodyshape. Not "using a previous model to create them". The next is how they play. Their differences aren't supposed to be massive(Ken's isn't even that massive, though borderline. He shares the exact same gameplay mechanic, and it's still noted it's mostly a conscious decision and if anything, is a bad example of what a general Echo would be). Chrom is really the best example of what we would normally get otherwise, at its peak, if we're not putting in really heavy oddities. Ken's an outlier. Daisy is too, as other ones have less arbitrary differences. Richter is a perfect example of "being reasonably similar, but having a notable difference", which applies more to the rest.
We also don't have much details either. One thing of note appears to be "same weight", but this is a coincidence(and if it holds remotely true, it's still a funny coincidence that Dr. Mario was a different weight from Mario at least once. Obviously this wouldn't be why he's not an Echo) at best and not actually told to us by Sakurai, but by Reggie. His is more of an observation. What we do know is that if moves become way too OOC(like with Isabelle, though do note she couldn't clear the "same bodyshape" criteria either), the idea can become scrapped.
Something else of note should be about a great example of one who may not have really been an Echo either; Pikmin & Alph. Unless the Pikmin(all of them) play and are the same bodyshape(or super close to, as Sakurai may make exceptions), they wouldn't be an actual Echo. But a regular Clone. If it was just Rock Pikmin, this could make sense. But if any others were used, the Echo label wouldn't work very well. The thing to truly remember is that the Pikmin and the Captain are the entire set of characters, not just the Captain. While it does seem arbitrary to why, this is actually counting how it works. The Pikmin are not actually accessories or summons. They're literally a part of the full character. It's like saying Kazooie isn't part of it, or Luma for that matter. Or even Nana, heh. Yes, they're part of it. So depending how Sakurai did it, Pikmin & Alph were probably not going to be given the Echo label to begin with. We also don't know if they were considered playable before or after becoming a costume. Just like Lucina, he was resized a bit to fit the costume. Unlike Lucina, the timing isn't clear to us. He might've been considered playable before becoming a costume, after all.