Here you are discussing about two different things: Flags and ratings.
I understand "likes" as flags. Flags are individual marks you can place on any object/content. This applies to "bookmarks", "thanks", "likes", "+1s", "pins" or whatever you may call it. These flags can only be positive, because they serve the only purpose to be counted and/or listed.
On the other hand, I understand "like/dislike" feature as a rating, like the one you have on phpBB's Ideas section. Ratings can be of many types (five stars, ten points, percentage), and one of them is "positive/negative". You can use this "+1/-1" rating to show the results in many different ways: Total positives, total negatives, both, or combined (combined=positives-negatives).
In this case, as this is proposed to be included in phpBB core, I would go for the simple flag option: "likes". That's the way other forums and websites work, including the ones mentioned in the first post (IPB, Xenforo, VB, etc.).
Adding negative votes or "dislikes" seems more appropriate for a different extension, maybe a post-ratings one with several rating options(+/-, stars, etc.).