Truncating the output of AES-128 I'd like to use the output of a pseudorandom function (PRF) as a one time pad.

I want to use AES-128  (as it's commonly used as a PRF). As we know AES-128 output 128 bits. 

Question: Would be secure to truncate the output of AES-128 and get only 32 or 64 bits of it as one time pad to mask some 32 or 64 bits value respectively?
What you describe is exactly using AES-CTR to encrypt a 32-bit or 64-bit message.  So, yes.