MacBook Air (2017) to an iMac 27" (2017) monitor

Physically, yes ... but will it work, no, if you are planning on using Apple's "Target Display mode."

Ref: Use your iMac as a display with Target Display Mode - Apple Support

Your 27" 5K Retina iMac does not support Target Display Mode.

That feature went away when the first 27" 5K Retina iMac came out in Late 2014. No Retina iMacs have it.

You could consider a 3rd party solution, e.g. Luna Display.