As notochord is mesodermally derived then it is prenst in diploblastic animals or only in triploblastic??

Dear student,
Notochord is mesodermally derived and organisms having notochord are called as Chordates. Organism without Notochord are Non-chordates. 

Notochord is present in Chordates only and some of the Characteristic features of chordata are:
​​​​​Bilateral symmetry, Triploblastic, organ-system level of organisation.
Yes, it is correct that notochord is present only in triploblastic organisms i.e Chordates.
Regards