KendoUI Mobile does not provide a 'special' styling for the validation messages. I also believe that the problem you described is not related to the custom validation rule. Does it appears with a regular required validation?
I am not sure exactly where the problem comes from because you did not shared the mark-up of your View. As an assumption, the issue might occur if the input is placed inside a ListView component. In that case the framework will wrap the input inside another html element which will cause the validation message to appear over the textbox.
Due to the smaller screen size of the mobile device, displaying the validation message in the 'default for desktops' way is not suitable. My recommendation is to consider displaying the messages outside of the ListView. This topic explains how to customize the validation message position.
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