The assessor is technically right, you shouldn't engage with the crowd. But, as others have said, sometimes it can be good!
I remember a County Cup SF I lined and I had a good laugh with the crowd throughout, resulting in a genuine round of applause from them, all in front of the CFA representative who said I'd enhanced the County FA's reputation with that small group, with one 90 minutes of action.
Equally, I remember a Contrib line, under assessment, where the crowd had decided that their 'banter' for the day was going to be aimed towards me for 90 minutes. An utterly hilarious 90 minutes, mildly encouraged by me, with the assessor complimenting my professionalism for getting on with my game appropriately, but not 'frustrating' the crowd, by ignoring them!
What I'm getting at, is there's a way to do it, and a time & place.
High-pressure game, directly in front of the assessor, stopping you concentrating on the game - no no no!
'Dull' game, action away from you, crowd trying to have some fun with you, absolutely do so!