charging my Ipad mini 4 with Iphone 12 charger, is it good?

If your talking about the brick, you acan use that for both devices, however if your talking about the cable, the ipad mini 4 comes with u s b c and not lightning like the iphone so you'll need to use the separate cable for it.

The way batteries work, basically anything that makes a battery hot will probably age it more quickly and it may lose its capacity prematurely. If the USB C 20W charger doesn’t make your iPad overly hot then it will be fine but I always use a lower power charger as my main charger so my batteries last longer, it seems to work too. If I’m in a hurry and need the device to charge quickly I’ll use a 20w charger just that time but apple will tell you its fine to use a high power charger all the time which is true but will reduce the lifespan of your battery. It’s kind of up to you, if you want fast charging, use the 20w charger and if you want the battery to last longer with less degradation, use the lower power charger either 10w or even 5w. There’s no right answer, just what you prefer.

.