Why my iPhone 12 mini battery maximum capacity is showing 90%

Batteries are consumable parts. With every charge, the battery chemically ages and is able to hold a lesser charge. This is not something you can stop and yes, the battery health in your iPhone will continue to degrade over time. As kb1951, correctly suggests a reduction of 1 - 2% per month is common and expected.

When battery health eventually declines to 80% or less, it will then be time to pay Apple to replace the battery in your iPhone.

Maximum Capacity will always decline. When the Max Capacity gets down to 80%, it's time to replace the battery.

You probably have about another year or a bit more on your battery.

You can expect a 1 - 2% decrease every 1 - 2 months.