Actually, his sweetspots and sourspots were pretty on par with Marth.
http://www.ssbwiki.com/Marth_(SSBM)#Moveset
http://www.ssbwiki.com/Roy_(SSBM)#Moveset
The fact remains that, while statistically they share very similar damage ratio output, in practice Marth is the superior character due to having the better spacing options. Roy
has to move in a lot closer to get the same kills Marth otherwise could, which is what left him lower-tier in Melee. Lucina, while her blade doesn't reach the percentage as high as Marth or Roy, still has options that Roy lacks due to being able to attack from safer distances while still achieving similar effect. Add onto the fact that Shield Breaker is a case of being less awesome but more practical in competitive play than Roy's Flare Blade and that she'll still have customs Roy will otherwise lack... Roy has more power than Lucina in theory, but if it's harder to land close hits with him and his match-ups suffer because of it, then Lucina is going to be the more popular pick in the meta. Not to say Roy still won't be everyone's boy, though, just that he's far from making Lucina "irrelevant".
Dark Dawn was all the way back in 2010, though, so what defines a character as being "irrelevant"? Lucina will still make an appearance in the most recent Fire Emblem game via Amiibo, Ice Climbers were in Brawl and have technically made a 100% attendance rate in their own series (not hard to do when you only have one game lol), and Mr. Game & Watch has been all over the place in games like the Wario series, DKC Returns, and Nintendo Land... All of this said, the fact that you're naming characters that have been in Smash Bros. as irrelevant makes
zero sense in the first place. Unless there's some greater meaning to the word, it doesn't matter if you consider them irrelevant when the fact that they're playable in Smash Bros.,
the highest honor you can possibly give to any Nintendo character, makes them anything but irrelevant.
It's not like someone received an early copy of the final build. Being datamined means there's bound to be a few homebrew-related quirks, and the Mario icon is without a doubt a placeholder in this case.