You would only Like (vote) a suggestion that you agreed with.regularly check and vote on every suggestion in the suggestions forum
I for one do regularly check both suggestions forums. Do bear in mind though that not everyone owning XenForo actually visits these forums on a regular basis and some only if they have a problem or even not at all.
Some suggestions for XFMG do have a reasonable number of Likes and these are clearly the popular ones. Others have very few or none. If you look at some of these, many are a bit niche and some are ill-considered.
Maybe the reason why many XFMG suggestions don't have many votes is because most people don't think the suggestions are worth adding? As a long-time user of XFMG, I do agree that some improvements would be welcome (Categories for Albums, the long-promised video uploads, adding tags and fields to the upload page). However, on the whole I do like XFMG the way that it is. I certainly don't want the radical overhaul and changes that are being suggested by some, which on the whole are those that don't actually own or use XFMG on their sites.