Can a Maxtor ATA133 HDD be fitted into a 2001 iMac?
Having looked at the iMac (Early 2001) - Technical Specifications and comparing them with the information found on Wikipedia for Ultra ATA/Parallel ATA I do not see any technical limitation on why it should not work.

What you need to be aware of, however, is that in case the bus speed is lower than the ATA/133 (e.g. ATA/66 or ATA/100), the hard drive would not be as fast as it could be.  Nevertheless, I understand that it will be operational with the larger size available to you.