The formats field is always sorted in alphabetical order.
If you want to distinguish MOBI from others, then in the current calibre you can create another custom column that has a value if there is a MOBI format, otherwise it doesn't. You could then sort on this column. The template would be something like
Code:
{formats:'in_list($, ',', 'MOBI', 'MOBI', '')'}
Another way is to search for format:=mobi and then sort however you wish.
In the next calibre release, you will be able to reorder the formats list to put MOBI in front. One template to do that is
Code:
{formats:'in_list($, ',', 'MOBI', merge_lists('MOBI', $, ','), $)'}
This doesn't work in the current release because the list is re-sorted after the merge.