Is a humorous "easter egg" in the academic homepage acceptable? This is my current homepage at my university's website, 
I've considered adding a little "humorous" thing to the homepage to make it less "boring" and more approachable (the "updated" version can be found here, click at the button on the bottom of the page - this is still not uploaded as my main homepage).

My question is, is something like that considered acceptable? will it leave any bad impression? Personally I find it quite funny and like small jokes hidden in serious websites / papers / h.w / books / etc.
The hard thing about using humor in a professional setting is that different people have different tastes, and so you're pretty much guaranteed that your joke will fall flat with a significant percentage of the people who interact with it.  That is one of the several reasons why it's generally inadvisable to use humor in an "unattended" situation like your website where you can't be judging and metering to your audience.

That said, the nice thing about a (non-offensive) little easter egg is that most of the people who won't like it are also people who won't ever notice it or click on it.  The sort of person who goes to click on a random odd little widget on a web page is the sort of person who is interested in easter eggs and wondering what they will find.  They might like it, and they might not care for it, but as long as it's innocuous, you're almost certainly not going to be doing yourself any harm.