What does "You feel stupid. You feel clumsy." mean?

It is a standard feature of roguelike games that you get messages describing the changes in your status and happenings around you. These can be cryptic to the obvious.

An example of the messages from Nethack (another rogue like game): nethackwiki Category:Messages. Another wiki with various 'you feel' messages: nethack.wikia.com You Feel

The two that you describe "You feel stupid" and "You feel clumsy" refer to the change in the status of intelligence and dexterity that you have had when putting on the ratskin cloak. You can see this also described in the description of the cloak: Dex-1 Int-1

Taking off the cloak would also be a status change regaining the attributes lost and you would get messages of 'You feel smart' and 'you feel agile'.

The ratskin cloak gives you -1 to dextery and intelligence. The game is alerting you to these maluses by saying you feel clumsy and stupid.

The cloak gives you negative intelligence and dexterity and the game is alerting you about it.
Source and all possible equipment alerts.