The main issue is that Merric doesn't use swords, so he wouldn't really work at all as an echo. He's a purely magical character.
In a perfect (for me, anyway
) world where Merric got into Smash, he'd likely have a set built around his legendary wind tome, Excalibur. His moves using the tome would do much more damage to airborne foes, just like in the main series, making him a potent anti-air focused character. However, he'd be prone to ambush, being fairly light and easy to knock away if the foe manages to get close, since he's canonically frail and fragile.
It's weird, too. Merric has pretty good credentials in a vacuum, but due to the nature of Fire Emblem's cast being ever-changing, he's a no-hoper.
+ He has a playstyle filled with untapped potential as a decidedly anti-air magical wizard.
+ He's the childhood best friend of Marth, the face of the franchise.
+ His design inspired lots of other Fire Emblem Mage units, becoming an archetype in the process. He was the first playable Mage.
+ Has a signature weapon to set himself apart from other units.
+ He's visually distinct from the currently-playable Fire Emblem cast.
+ He's also voiced by Yuri Lowenthal, Marth's voice actor, lowering the cost of another VA.
+ Due to becoming Marth's brother-in-law, he's related to three of the current playable Fire Emblem characters (four depending on who Robin marries, five if Corrin marries Odin).
- He's relatively minor in the grand scheme of Fire Emblem.
It just goes to show how specific series' barriers for entry are taller than others. If Merric had these credentials in any other non-Pokémon franchise, he'd have been in the game already.