Yes if the trigger is executed; otherwise. No. The default value is No, unless the FileFormat Property is set to a value of UPXML for User Portal dataport. In this case, the default value of the CallFieldValidate property is Yes.
Imported fields are validated in the same order that they appear in Field Designer. The code in the OnValidate (Fields) Trigger might initialize other fields in the record. If this is the case, investigate whether the order that fields are validated is important by looking at the OnValidate triggers of those fields.