No, it wasn't always ridiculous. At first, it was about a bio-experiment gone wrong, albeit partially ridiculous, though still believable in a purely video game sense.
The series started getting numb after Resident Evil 2, and then with #4 where they held nothing back in the campy horror story department.
Resident Evil 5 is a complete departure into the ridiculous. With cheesy anime inspired characters and a pseudo-MGS plot. The entire time I felt like I was playing a horror spin-off of Metal Gear Solid, without the stealth. Though, not like MGS4 had any stealth going on either. It's as if all games are being assimilated into an amorphous action game substance.