I like the concept, but I would like to see the surface more detailed, riddled with cavities and irregularities. The matter should be largely more brain-like and less muscle-like (it currently looks more like the latter).
Anothers thing that bothers me is that the overmind has no use for those fangs (they suggest a predator lifestyle, which is generally hard to imagine if you are a big rooted blob of brains in charge of an alien empire).
In my opinion, the teeth and the oral cavity could altogether be replaced with some sort of a (perhaps gill-like in appearance) vocal apparatus (usually employed as a backup short-range signaling system in case direct telepathy fails or so).
The vocal apparatus idea could also see some usage in the singleplayer plot, as I imagine it. For example, we already know that for whatever reason the overmind cannot assert control over humans' minds (not a single case reported in the history of Tremulous, unless I missed some particular piece of fanfiction), and no common alien classes seem to be capable of truly intelligent vocal communication (which they don't really have a need to — they are combat troops, after all, and should have as obvious a combat role as possible, even if that includes baby polar bear screams), therefore the overmind's secondary vocal means of getting his point across could be useful for, say, declaring «resistance is futile» a thousand times into the comm screens of the human kind (seeing as that is a popular sport among extraterrestial intelligence).
Eyes, as gavlig highlighted, make little sense on their own too, but I propose we leave them as-is. They make determination of which side is «front» much easier, not to say that the overmind as we see it might not always been the same organism — evolution is key. The eyes could be a leftover feature of the overmind's evolutionary process, a parochial back from the days it was a more or less independent lifeform; as an example, see our own body — our foodway crosses our airway and there's no particular reason to it, it's just a minor enough annoyance for it not to be counted against other things that make humans humans so it did not get washed away in our evolution.
tl;dr I approve with minor changes to construction