Where is the irrationality here exactly? Unattractive people are not attracting as much site traffic as attractive people, thus decreasing their odds of finding a partner... isn't that exactly what being unattractive means? Why would a rational person seek out an unattractive partner when attractive ones are available, especially given what we know about (typical) humans' sexual preferences.

I'm also not sure it's particularly rational to ignore looks of potential partners; we tend to value looks quite a bit in their own right, not to mention the substantial effect of a partner's looks on our social status and that of any future offspring. Unless you don't actually have a choice, i.e. you're settling, why not aim high initially? Especially with the low entry costs of online dating there's no reason not to take the risk and message the most attractive people available.