Does the value of someone's report go down if they report players often?

Only if they are false reports.

If the reports are accurate, there shouldn't be any drop in value.

but there's no tribunal so its up to the gods to figure out if they are false!!

Number of reports won't hurt you, but a large amount of false reports will reduce the impact of yours.

It's like crying "WOLF!".

It does, actually :D