I have been doing a research on the subtitle translations of the American Tv show called "It's Always Sunny in Philedelphia" and faced an instant where Frank mentioned having a mind as sharp as a thumbtack. I know that there are idioms as "having a mind as sharp as a tack/razor/steel trap" along with the old ones like "as sharp as a needle or thorn". However, I have never heard "having a mind as sharp as a thumbtack". Although "as sharp as a thumbtack" and "as sharp as a tack" sound similar, if the native speakers have never heard of the idiom "as sharp as a tack", that would be an important detail to shape my work since it would suggest that Frank made up or changed that idiom. Thank you in advance.
The idiom is "sharp as a tack":
Very intelligent and perceptive
I would imagine "sharp as a thumbtack" would be heard as a minor variation, unless the context of the show made it clear they were flipping it from a compliment to an insult.
Although if they were trying to flip the meaning I'd expect something much more obviously not sharp, or maybe something absurd like "sharp as a marble".