I do believe there was one discussion regarding customs, but
Firstly dude, if she's fun to you, use her. Don't rely on her tier placement ^^"
Ok, now personal opinion aside, by the 1.04 Japanese tier list, she is next to bottom (with Marth only a few places above her, but also bottom tier). This may also be due to a lack of Marth/Lucina representation on that end, but who knows? America side tier list wise, I don't believe there is a set tier list yet. People are still discussing and debating it, so hard to say.
In terms of viability, I am assuming you mean tournament wise. By the numbers, she is decent and can do pretty good work in the right hands. I won't compare her to Marth since you only asked about Lucina, but there are threads for that which have gone full circle. So please do not ask about it, read what is there (particularly Shaya's post on their differences), and decide for yourself.
I'll stress one more time that if Lucina is fun for you, use her. No matter her tier placement, even with the odds stacked against you if ever, you can make her work if you learn how to use her well.