Google's John Mueller was asked if using WordPress themes that are used on many websites can have a negative impact in search rankings and SEO. John Mueller responded on Twitter, no, he said "popularity of a theme has no effect on SEO at all."
Here are those tweets:
Popularity of a theme has no effect on SEO at all.
— 🧀 John 🧀 (@JohnMu) October 11, 2021
If you hear such generalizations, ask them "why".
Yea, it is always good to ask yourself why when someone tells you something.
We do know that changing your WordPress theme can impact your rankings, that makes sense when you ask why. We also know Google does not give WordPress preferential treatment - but in terms of a theme being over used, - nah. Content can be overused and duplicated across the web and good luck trying to rank for the same content as several other publishers. But themes? Nah...
Forum discussion at Twitter.