This may always be one of the most difficult areas for someone with ASD, because you aren’t sure if they are looking for help with a decision (in which case negative feedback is OK), or for affirmation (in which case negative feedback needs to be avoided at all costs).
I suggest trying some more neutral responses when you can’t say with full honesty that you like something:
“I don’t feel qualified to say anything about this art style”
“I haven’t much experience with art”
Or turning the conversation so you don’t have to give an opinion:
“What would you like to do with it?”
“What drew you to it?”
“Horses! You really do love horses, don’t you?”
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).