Not easily. We chose the official type names because they're the same for all .NET languages, so we could store the type info inside the mapping information in the project. So the code generator emits the .NET type name as-is.
If you REALLY want this, you can alter the TDL interpreter sourcecode and change the case clause for TypeOfField at line 1591 of TDLInterpreter.cs (v2.5 code).
It doesn't generate nullable<T> though, that's controlled inside the template: See for example line 621 of entityIncludeAdapter.template.
I wonder why you would spend time on this though, as it won't give you any extra value, only extra maintenance: int is a synonym for System.Int32, short? is a synonym for Nullable<System.Int16>