Let me tell you, I've played my fair share of Sona- about 230k worth- and she's almost never been meta (aside from before her rework, i suppose). I think there was a point where she did quite well because the game was somewhat balanced around tanks and not damage dealers, so she was able to fit right in and not die in .5 seconds all the time. There was even a time where Sona mid worked and was fun.
Otherwise, it's been a rough ride for her. The assassin debacle, the AD itemization changes, the focus on snowballing and blowing up your opponent? Heavily working against her. Not to mention she's not exactly a successful rework in the first place.
The only thing that really helped her were the mana regen changes, support item changes, and the support keystones, which is a pretty flimsy bandaid on a broken arm.
To basically sum this up, she's not bad, but she's not good. She's heavily outclassed by virtually any other support and has few qualities you'd want to pick over other more focused supports (e.g, Soraka for healing, Janna for disengage/chase, Lulu for interrupts and counter engaging, Karma for heavy poke and single target focus, Morg for cc lockdown and protection against magic and cc, and even Nami for being what Sona is but just... better at it).
She's too slow, too frail, mana hungry, has mediocre buffs, heals, shields, and damage, and is basically a medium range poke/ult bot. Press Q, try to not die when using her empowered AA, throw in the occasional shitty heal or speed boost and the VERY occasional ult (because its cooldown is so. fucking. loNG.) and wooooooOAAAAAAAAAAh look at her GOooooOOOOOOooOOOooOooOOoOOOOOoooooo