The height of the area of the metadata bar which is visible without scrolling is defined in the file 'resources/mmv/mmv.variables.less'. Changing the corresponding entry to
@metadatabar-above-fold-height: 0px;
makes the metadata bar disappear at first sight. Its content remains available via scrolling, though.
This is the perfect solution for people who do like MultimediaViewer for the functionality it provides but dislike the visual design that features font sizes that attack one's eyes with a yell.