As of this time, QuickBooks Online doesn't have an activity log that shows estimates viewed by your customers.
The viewed status will only show on an emailed invoices that was already seen by your customer. But I want to let you know that we're taking note of your feedback and suggestions to improve the experience we're providing.
In case you need tips and related articles in the future, visit our QuickBooks Community help website for reference: QBO Self-help.
Fill me in if you have any other questions managing your estimate. I'll be around if you need any help.