Every now and then, I find comments which I like. Were I on facebook, I would put a like. Here on SX, "like" is not an available feature. The closest I can think of is upvoting. Sometimes I do, sometimes I don't. I have seen humorous comments with many upvotes, so I guess this is not too bad an idea. However, upvoting should be used to mark constructive / useful comments. Some comments I would "like" are decidedly not constructive nor useful. Is it advisable to upvote such comments or should I just leave them alone?
Depends on context, as usual. The main thing to consider is that comment votes are used to determine which comments are shown on page load, i.e., are in the face of every visitor from the Internet who ever looks at the post.
On the main site, $5$ top voted comments are shown; on meta sites, $15$. So, if one particularly funny comment gets upvoted to the skies, no big deal. If there are several, and they are crowding out less funny, but more useful comments, there is a problem. Withholding upvotes, or flagging for a moderator to delete obsolete / chatty comments, is appropriate then. For more, see A guide to moderating comments.