Does the team get points if the word is singular but they guess plural?
I agree with @MazelTov that many people probably allow derivative forms as successful guesses, but I do not believe that was the intention of Taboo's designer. Some games may explicitly allow forms of the guess word, but Taboo is all about the WORD. It should be an exact match.

For reference, the rule for a successful guess is very concise when it refers to guessing the correct guess word:


  
  SCORING A POINT Each time a teammate shouts out the correct Guess word, the Clue-giver’s team scores a point. The guessed card remains
  on the easel. The Clue-giver quickly draws another card and puts it on
  the easel in front of the already-guessed card(s). The number of
  guessed cards in the easel is the number of points the Clue-giver
  earns for his or her team.
  


That said, some people find this pure interpretation to be frustrating in actual gameplay. With that in mind, I think allowing for singular/plural is probably more acceptable than other derivatives (mountain/mountainous), but anything other than the actual guess word is a house rule. Naturally, house rules are intended to improve the gaming experience, so it's important to announce such rules at the start of play.