MediaWiki doesn't require PHP to have support for mbstring  (it has fallback functions to handle that)
The HtmlFormatter class, part of MediaWiki core in MediaWiki 1.22 that's being used only by Mobile Frontend for now, is accessing "mb_" PHP functions that aren't being defined as fallback, giving fatal errors when PHP doesn't have mbstring support.
The missing function is mb_convert_encoding
We should provide a fallback function for it, or check if ( function_exists( 'mb_convert_encoding' ) ) before using it (as it's being done in DatabasePostgres.php line 437
It currently breaks Mobile Frontend on installations without mbstring