Talentwise she's in the top 20%. But the more important thing is how marketable she is. You can almost see the Pound signs come up in Simon's eyes when he sees her come out in an outfit that shows how good looking she is. It's a slightly dangerous thing sexualising a barely 17 year old, so the show has had to be careful, Even her Essex girl angle works well here, because she's not TOO Essex--rather than looking like the usual Orange skinned overly made up Essex bird, she's got enough classic English Rose in her to play the contrast.
It would make for a nice short-term marketing success regardless, but fortunately she can actually sing reasonably well, and more importantly has charm and stage presence.
As for her coming off as "bland" to some? It will depend. This is equivalent to the situation they had on the US' American Idol when Carrie Underwood won. Beautiful young white girls come off feeling generic. Even a bit of Essex doesn't change that. But that's a longer term problem not a short term one. In the short term, she's super-marketable.