In terms of Smash? People complain about too many FE characters and too many FE characters that use swords way too much. Yet they opt for Lyn to be playable, sometimes even saying she should replace Corrin (who is more unique than Lyn would be BTW) because "series history". Or more accurately, their own personal preference.
In terms of Fire Emblem? One of the many terrible arguments the Fire Emblem Elitists use in their pathetic attempts to make it sound like there's nothing good about Awakening and Fates is "too many sexualized waifu's". Which is EXACTLY what Lyn is. Yet she gets a pass because she's from the older games.
I don't hate Lyn. But I stand by the stance that she is incredibly overrated.