Here's what "mind share" is:

"We are competing for 'mind share' with the public, and it is becoming more and more difficult to grab mind share," she said.... which involves, she said, "committing a healthy percentage of your revenues to keeping your image before the public." (http://www.mediainfo.com/ephome/news/newshtm/stories/072297c...

In other words, how much the product stays in people's minds.

It appears you can use a direct translation, "cuota mental" in Spanish, for example:

En lugar de impulsar la cuota de mercado en el sentido tradicional, estos productos interactivos impulsan la «cuota mental» de un grupo de clientes superimportantes para la empresa. (http://www.andalucia-e.com/cursos/empresa_en_www/1.htm)

Finalmente, el emprendedor debe saber convencer a sus inversores de que lo importante no es ahora vislumbrar retornos a corto plazo, sino conseguir una posición preferente, una cuota mental del mercado de internautas, que dentro de unos años (2,3,5...) se convierta en una base sólida de negocio (http://www.emprendedores.cl/articulos/info01.htm)

Hope it helps.

Accepting the sound explanation given by luisafern, this version seems to me more idiomatic...