Tbh, for me, it's not a question if it is allowed or not, but more about common sense...
Upvotes are there to support others and appreciate, and that's how I try to use them... Upvoting comments of people who engage with my posts/comments is something natural, as those people decided to spend time with me and my content, over doing something else... At least that I can do for them is to upvote their comment...
It is allowed on a technical level, but as you said its meant for others.