Current dashboards can be scaled easily using the 'Automatic' sizing. However, this doesn't resolve the issue, especially for smaller tablets and phones and can lead to formatting differences between devices.
Being able to define media queries or built-in responsive tiles would be a great addition.
-->Community Manager update [Ciara Brennan]
Specifically related to automatic dashboard phone layouts, this feature was released in version 2019.1
"Authoring mobile dashboard layouts just got easier. Tableau will now automatically add phone layouts to new dashboards, with ability to further customize them for specific use cases.