That's a difficult question in terms of how different people might perceive it. On one hand your performing a tarot reading regarding someone else without their prior knowledge or consent to gain insight into their personal feelings. On the other hand your divining information about your potential future and forces that could affect you. In the end tarot reading is a talent or skill which in itself cannot be ruled by morality or ethics very well as it is how you use the knowledge you receive that matters. I don't see anything immortal or unethical about wanting to know how someone feels about you, but remember the future isn't set in stone nor is tarot reading an exact science so be careful about making snap decisions based solely on the cards.
Ethical is subjective, yes; I also think it's nothing to worry about, no big deal. I mean, if you were about to cast a love spell then you may needed to think about it a bit... But I find nothing wrong with that.
I like to think the tarot cards will tell me what I and the person I'm reading for need to know, to ask it for more information beyond that may bring the risk of breaking the connection and energy you have with your cards.
Personally I don't do readings for a third party ie. how someone else may think about you, simply because I see it as forcefully prying into someone's privacy and delving into their private lives and that itself brings about a lot of negative energy.
You also have to understand and remember that tarot archetypes are deep insight into YOUR own psyche and human psychology, it can also be about your wider world and yourself in society but overall its a look into yourself and so the tarot can not give you a very meaningful answer to your questions regarding how someone else thinks about you.
To answer your question, I do not think its ethical to ask the tarot what someone else thinks or feel about you because of the breech of privacy but also because there is a limitation of how YOU will interpret the tarot and not the wider issue at hand.
However you could reword your question to fit your purpose better.
Also remember the tarot will give you precious insight and answers to your question regardless if the answer is not the answer you want, you should take the answer the tarot has given you and meditate on it. DO NOT fall into a trap of re-asking the tarot the same question repetitively just to gain the answer you were looking for.