Is Dungeon! biased towards the Cleric?
I have not crunched any numbers for "proof" but experience and a quick sort through the monster deck would tend to support your supposition. 

Rogue hit numbers are notably lower than the Cleric's and their secret doors special power is not good enough to compensate, especially given that it's not reliable and can be replicated with certain treasure cards.

Fighter hit numbers are only notably higher than the Cleric on low-level monsters. So it's more a case of being more reliable than really adventuring at higher levels. That doesn't seem a good enough trade off for the extra 10k gold.

Wizards can do much better than Clerics, but it's a crapshoot: draw monsters that happen to be vulnerable to your spell selection and you'll win easily. Don't, and you're monster bait.

But to be honest, you don't play Dungeon for the sort of strategic gameplay where balance is important. You play it for laughs, for stories and for being about the only family-friendly boardgame with an unashamedly geeky theme.