It is barely visible in the heat of a fight. I bet those of you who don’t play tyrande don’t even remember what the hunter’s mark looks like because you’ve never seen it before.
I did a simple google search and found a reddit thread with close to 800 upvotes for this change: https://www.reddit.com/r/heroesofthestorm/comments/8gpedj/tyrande_really_needs_a_good_visual_indicator_for/
It’s pretty important to signal to your allies to focus the target, so I don’t know why the visuals for this ability is almost invisible, especially because there are multiple tyrande talents that buff the mark (e.g. cause allies to heal when attacking marked enemies)
The funny thing is, based on the reddit thread, it was once (in the past) more visible but was changed so it became invisible. Really odd change if you ask me…