Is having a technically great voice overrated?
No. If a voice is technically great, you can't overrate it by simply claiming it to be what it is. Firstly I'd prefer not to tie in this phenomenon of a technically great voice with TV 'talent' i.e. entertainment shows. That could be seen as a disingenuous association. Luther Vandross, Gladys Knight, Otis Redding, Lalah Hathaway, Marvin Gaye etc. etc. don't and didn't have anything to do with The X-Factor.
What makes an artist interesting? For me, it is that they make great records; as simple as that. I'm not interested in an artist because they project some image, because they are quirky, or eclectic, or wear certain clothes, or are a bloke in make-up, court controversy, constantly reinvent themselves etc. They might do any and all of that but if their music doesn't cut it I wont care.
Great voices need to be taken care of, they change over the years, singers' interpretations mature. A vocalist is an artist, their voice IS enough. If their style of music isn't your cup of tea, that's fine but there's no need for this sort of inverted snobbery where for instance, any four-piece rock act that happens to knock out their own half-baked material is somehow far better than say, Ella Fitzgerald. (Not directed personally at the OP by the way, I just feel there are too many who take this sort of view).
They may or may not compose, or play, or engineer their records and just as you feel that they record music that "serves as a showcase for their warbling" (which is rather prejorative) I could easily state that many acts record only those songs that won't challenge their meagre vocal capabilities or show up they limited instrumental proficiency.
People don't seem to focus on drummers or guitarists who don't compose music or lyrics or producers who don't play instruments or song-writers who don't sing yet they feel it's no big deal to be a fantastic singer if you don't do something else on the side.
Of course there are mediocre vocalists/stars who only front a record and also mediocre acts who write play and produce there work; I'm just kicking against the generalisations that are made about music. Most records are made by with the assistance of others and not as the work of one and as many of us know, 'stars' will get credit / take credit for things that they didn't do.