I definitely think mixtes can be collectible. As others have noted, there is a demand for mixtes, especially around college towns, and high-end ones are not too common.

Some day I'll finish building this for my daughter. Double-butted 531 tubes & Campagnolo dropouts, and it came well equipped with a smattering of Campagnolo NR, Stongliight, and other high-end vintage parts. Still needs paint and it's been put on the shelf since I got her another beater bike in the meantime. Good thing I bought it while she was too small for it, 'cause it'll fit it by the time I'm finally finished.