Is the sentence "You have to be ascetic about eating junk food" correct?
Ascetic: Practicing severe self-denial
an ascetic is one who is characterized by the practice of severe self-discipline and abstention from all forms of indulgence, typically for religious reasons.
an ascetic life of prayer, fasting, and manual labor
The word denotes and connotes a lifestyle, not individual choices here and there inclined toward sacrifice.
As such, You have to be ascetic about eating junk food is a misuse, as it is a choice to eat healthfully, not to meditate, sleep on stone slabs, pray, work and eat less than one wants to.
It's grammatically fine, but the meaning of ascetic is much stronger than someone who would never eat junk food. An ascetic wouldn't eat much food at all, abstain from sex, and generally forgo most or all material comforts.
As such, you can't really be ascetic about eating junk food, or to the extent that you can, you would just use the expression, "never ever eat junk food".
I do not believe you can practice severe self-denial while eating. There's even an idiom,
Would you both eat your cake, and have your cake?
You're using it kinda verbishly, where common use is more noun or adjective.
Examples that sound better to me (western usa, fwiw):
"Jane's an ascetic. She doesn't eat junk food."
"Ascetics abstain from junk food."
"Extreme fasting is part of our monastery's ascetic practice."
"Asceticism is the practice of rigorous self-denial, typically as a spiritual discipline."
Others have mentioned that your use is wrong because it implies asceticism is casual. While yes, asceticism is defined in terms of harsh self-denial, it's the least problematic thing about your sentence. My first example is similar to your malformed sentence, and I feel it invokes humour by insinuating something as deep as asceticism could be achieved by simply avoiding junk food.