The K2 item form's CSS is probably being overridden by your template. Also, make sure that you don't have an obsolete (=old) itemform.php override inside your template's html/com_k2/default/ folder.

Look inside /templates/YOUR_TEMPLATE/html/com_k2/. If you don't find anything, it's possible that your template is enforcing CSS that breaks the layout of that page. In that case, you can simply modify all "edit" or "add" links and append "?template=protostar" at the end to enforce Joomla's default frontend template just for the K2 item edit form.