Can items and Tokens still be claimed from the Mansion even if the character is defeated?
You would apply the effect of any [Token][Item][Event], or any other Non-Infected card when it is revealed, before defeating or failing to defeat the infected. You resolve combat only after applying the effects of the Non-Infected cards.
The official Rules say:

Non-Infected Cards
Sometimes Events can be found when Exploring the Mansion instead of an Infected. In this case, When the Event is revealed, perform the effect listed on the card.

There is also the Comprehensive FAQ (which appears to be answered mostly by Alex_Bandai, who I assume is a Bandai Representative):

If I am defeated can I still collect Tokens or Items reveled?
Yes, you collect all tokens and items as soon as you discover them.