The value of $wgSitename
can only be about 20 chars before things start to be ugly. On mobile devices only 10 chars are allowed for the same reason. You can see the issue here. The Foreground skin solves this issue by providing a configuration prameter "wikiName" allowing to override the site name just for the display in the top bar of the skin. Perhaps this is something to considered here too.
Topic on Skin talk:Poncho
This is still an open feature request for version 1.5.0
Thanks for the reminder. This week I've been actively working on the Poncho skin and will release a new version in a couple days, including a fix for this. I created a Phabricator project for Poncho and a task for this, see https://phabricator.wikimedia.org/T226717 Let's continue there, cheers!