They aren't even self inserts, since they have their own personalities, defined backstories, etc. The issue is a bit different for each character, but i'll try to explain:
Robin: Despite utterly failing as a self insert, Robin is a pretty likable character. He's got great supports (
his support with Virion is arguably the best in the series), he's a good contrast from Chrom, has good story moments and is laughably overpowered as a unit. His failings come from the fact that they lean a bit too hard on him being "special" and he utterly steals the spotlight from Chrom, but it's not like one of his character traits is literally "people are drawn to him." In the end, Robin's a pretty decent FE character and easily the best avatar in the series.
Corrin: Corrin is also not a self-insert, but unlike Robin, her character is completely devoid of any ambition. Her character traits are extremely generic (one of them is literally people are drawn to her), her personality clashes with her character set up (she's a conversationalist despite being isolated for most of her life...) and her role in the story is... well it ranges from basic (Birthright) to outright awful (Conquest). Corrin is just massively disappointing. She has SO much going for her and they somehow manage to completely squander it. But she's a cute dragon girl and her moveset in Smash Bros is A+ so it's aaaaaall good.