Rule #1 of Careers: Bring your sense of humor.
Rule #2 of Careers: Bring your wallet.
I don't think it's an issue.
First: it's a funny message meant to diffuse the fact that an error happened. Humor can sometimes help ease tension created by a frustrating situation.
Second: who wants to be hired by a company that puts humorless people in charge of hiring you?
Third: if the errors happen frequently enough that it somehow tarnishes the site, and therefore the people who use it, then there's something far more wrong with the site than the 500 error message.
Fourth: have you even seen the monstrosity that is other employment sites? I'd far rather belong to a clean, slick, non advertising-coming-out-our-ears website that has a funny error message than any one of the hundreds of ugly, ugly, ugly job sites that already exist.
Fifth: LOLcat is now an established meme that anyone who uses the internet daily understands. It has already reached the corporate boardroom. In fact, if it WEREN'T a meme associated with all the stupid-funny lolcat image macros out there, I don't think anyone currently complaining would have an issue. Would it matter if it were one of Google's pigeon rank pigeons with the same text? The fact that it's associated with LOLcats is the problem for people, and that they feel lolcats themselves are stupid.
Bring teh funneh.