using 2017 imac as external monitor

Not using Target Display Mode because it is no longer supported.

Use your iMac as a display with target display mode - Apple Support

Check out third party solutions like Duet Display or Luna Display.

Or just use an aftermarket external display with the MacBook Pro.

Use an external display with your MacBook Pro - Apple Support

+1 No not unless you use a third party solution like Luna. Personally I would invest in an external display. Why, cheaper as it does not require expensive software, simple set up, dead reliable. If you are in the USA go to your local Costco or any number of resellers to find a display that suits your needs.