If the instagram is JUST for the comic, then posting just the comic is fine, but on its own it may not get a wide audience nor will it generate a lot of interaction. If you're really insistent on it being a comic-only account, Make one where you post those other things, because Instagram is about realness and community first.
To boost engagement on the comic account, maybe try asking a question in the comments? or maybe posting it to your main accounts story.