As far as I know, no one has purposely targeted PokemonDb for hacking.
However, there are hundreds or even thousands of automated programs out there (spambots) that try to log into any website or server they can find on the internet. If you don't have good security they can try many passwords per second and over time they might guess the correct one. If they managed to log in they could potentially put anything on the site (i.e. hack it).
Luckily I have many security measures in place, such as spambots getting banned after a few failed login attempts. Two have already been banned this morning.