Is cheating allowed in Munchkin?
According to the very old rules from the beginning of munchkin you could cheat if the other players didn't catch you.

Likewise, you may also use only one headgear, one suit of armor, one pair of footgear, and two “hand” items (or one “two hands” item), unless you have a card that lets you cheat or the other players don’t catch you.

That was in an earlier version of the rules and doesn't appear in the newer rules, so probably a no. On the Munchkin FAQ the fifth question under General Questions is

Q. But don't the rules say it's OK to cheat if you don't get caught?
A. Not anymore, and even when they did, it was a joke. If you want a game where cheating is in the spirit and the letter of the rules, try Illuminati.

So again no.
Anyways, cheating is breaking the rules and makes the game not very fun, so I'd say no to cheating.
If you want to read those rules, I accessed them from https://boardgamegeek.com/filepage/3069/rulespdf. You need a boardgamegeek account to download them. To find that specific passage, hit Ctrl+F then write &quot;cheat&quot; in the box that appears.
Also see this old cheat card: