You can validate that yourself by Googling some "Tattoo" related search terms and analyze the results.
If you see a lot of LOW AUTHORITY websites on page 1, then I would cross out YMYL.
From my quick analysis, I see a lot of low authority websites ranking for "Tattoo" related terms.
For example, take this keyword which has 700 searches/mo based on AHREFS:
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Those 2 results show up in the top 10 search results:
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As you can see from the MOZ DA listed below those 2 SERP results (Using the MOZBar Chrome extension), those websites hold a DR of 8 and 9 respectively.
On the other hand, if you search for YMYL related keywords in a niche like "Pregnancy", you are not bound to find any website with DA of 20 or lower.
I could be wrong! But this is how I personally like to validate vague niches whether they are YMYL or not.