iPhone Outlook app still fetches mail when in Low Power Mode, while the default Mail app doesn't
If you turn off cellular data (open 'Settings' and select 'Cellular' then turn off 'Cellular Data' option which controls all the apps) this will prevent Outlook from fetching mail.  Yes, it will disable all other cellular data to all other apps, but there doesn't seem to be any other way to disable/change Outlook behavior.