It wasn't that the batteries were "substandard". I'm sure they were the same technology and quality as the standard iPhone 12 and 13 batteries. It's just that they were compressing the same hardware into a smaller form factor and, therefore, a smaller battery.
The only thing that used less power on the mini was the smaller screen, but that doesn't save enough power to make up for a physically smaller battery.
As a 12 mini user daily since it came out in 2020, I've only just now started to hit any noticeable battery dip (~85% after almost 5 years usage). It's still pretty solid on a daily basis. On very rare occasions, the smaller battery has required charging before evening due to excessive photos taken and/or nav without a plug.
FWIW, I will probably replace the battery by end of year, or next, and keep it going as long as I can... I refuse the massive "normal" phone size.
Imma big Mini fan but the battery is sorely disappointing after a year on both my 12 and 13. It's especially true in daylight when the display goes max; walking the dog for 15 minutes sees a 10+ % battery drop. It's nuts.
That said I wish they'd release a thicker version to compensate
There are a lot of people who probably would've bought the mini but instead of opted for the SE because battery life degraded so quickly.