Simon, we are in the process of developing a Mobile Mashup Builder. You would be able to create, modify and load Mashups with ease (and with minimal footprint too!) as this is pretty much optimized for mobile phones. I don't have a tentative release date yet, but I'll let you know once I have more information.
Out of curiosity, coming from a Mobile ennoblement platform product company, what would you mark as missing in the existing Mashup composer from ThingWorx? For example is it a designated designer which is specifically targeting mobile devices (to improve the scaling / layout) only? or things like richness of Mobile widgets? Flexibility of creating a full app with nicely defined process? etc.
Also, assuming a nicely scaling Mobile solution, is there a value to a visual designer which the end-users or the business-users can quickly and dynamically create/customize their mobile / desktop UX for IoT using ThingWorx based on needs? (assuming it does not require any development cycles).