Kinda of. But the mini was released at a terrible time to get accurate data. The iPhone SE had launched 6 months earlier and had been lauded as the phone to get if you wanted a small iPhone. People who bought the SE were not ready to upgrade straight away to the mini. I suspect if the mini launched today it would see (a little) more success.

I'm sure Apple, and most other smartphone companies who have released a smaller phone before, are also at least as capable as us at accounting for launch timing in their investigations of what causes a phone to sell well and what doesn't.

Even outside of iPhones there isn't a large market for small phones. It's a niche market