If after istikhkara your gut instinct is still no, then that is the answer to your istikhara.
I'm a little older than you and I will say this much, in matters regarding matrimony don't be pushed to say yes. It's your life, you will be spending ever after with this chap and if you don't like him then stick to your guns woman!
It doesn't matter if your mother thinks he's the Yusuf (as) of our age, you dont find him attractive then that's fine, he is not for you.
And don't put yourself down either, I'm sure you are lovely.
I'll tell you something, when I got married I did try telling my mother I didn't find the guy attractive, my mother replied well your nothing to look at! Which actually is not true, I am actually beautiful! I wouldnt say this if it weren't the anonymous board
It transpires however that my instincts were correct, my marriage was horrendous for its duration and the man I married proved himself to be unworthy of me.
In matters of marriage always remember that it's going to be you waking up beside this person for the rest of your life, if you dont want that, then dont be bullied into it.
Do keep doing Istikhara tho, and pray to Allah for a good and worthy spouse inshallah.