Is it bad that I don't wanna talk to my aunt because she was brutally honest with me?

Oh get over it! She’s that way because she cares. Sometimes the hurtful truth is what we need to get our asses out of our heads! Sugarcoated things won’t help you grow as a person. You need to feel pain to grow.
Wait is this me talking? Wow

Sometimes being blunt is exactly what you need, and sometimes it isn't. What I've never needed personally though is someone who tries to say all the same things but in such a sugar-sweet way that I get confused and wonder what they're on about. Only you can decide whether the tough love approach is the one for you but to rule her out completely for it might be going over the top a bit. No matter how blunt she was at least you know she's not gonna lie to you and that alone is worth it's weight in gold.
It also might be the case that being older than you she's more set in her ways and would find it difficult to change. While you, being younger, might be able to adapt to change a bit easier.
I'm not saying you should embrace her on the spot but these are some of the things you might like to consider before you make your decision.
And good luck btw. I'm overweight too and wish to god I wasn't cos my spine just ain't gonna take much more.

PS. I'm not convinced you need to "like" her in order to accept the only sort of help she knows how to offer.

Should have not been your aunts business