Reviews allow "helpful votes", but I would like to also have this functionality for comments.

In my case, I'm using both reviews and comments, but I'm using the comments section for "tips". So let's say someone doesn't want to leave a whole review, but has a "tip", they can leave it in the comment section. If the tip is helpful, they should be eligible for a "helpful" vote.

Is there a way to accomplish this?

Sounds like a mod could do it since the functionality is there. Try requesting one.

The Like button is there FYI.