If I make my mind up to answer a HDIL question, I'll tell them the truth from my perspective, whether I think they are attractive or not.
If I call them attractive, I usually try to give a reason why like "you have pretty eyes" or whatnot.
If I call them unattractive, I list what I think they can do to improve, like saying "if you apply less makeup, it would look more natural and make you look more attractive" or "you would look better if lost some weight." I figure that calling them straight up ugly doesn't really help them out-- it only hurts their self esteem. The HDIL questions I've answered where I gave suggestions receive generally positive reactions from asker on the whole.
However, offering suggestions to improve is beneficial to them. If they are asking a HDIL question, I would hope they want honest ratings and stuff, not just everyone saying "you look 10/10" when they really don't mean it. I don't believe it is right to lie to the asker and give them false confidence. False confidence can be shattered and create more issues.
If they are actually genetically erm... not very attractive and there is nothing that can really be done about it, I usually say something along the lines of... "you are not my type. However, everyone has different preferences. What isn't my type is sure to be someone else's." That way, I make it clear to them that while I may not be attracted to them, it doesn't mean that they have no hope.