Get Access Request Comments in Powershell script

Try this:

$sReader = New-Object System.IO.StringReader([System.String]$requestString);
$xmlReader = [System.xml.XmlTextReader]([sailpoint.utils.xml.XmlUtil]::getReader($sReader));
$requestObject = New-Object Sailpoint.Utils.objects.AccountRequest($xmlReader);
$comments = $requestObject.AttributeRequests.Attributes.comments