iPad screen dimming or changing brightness while charging, anyone else?

As described, you have encountered normal thermal management - typically encountered in high ambient lighting conditions such as when using the iPad outside, or in high ambient temperatures. Combined, using the iPad outdoors in high ambient temperatures - in particular when charging - it is relatively easy to encounter thermal management at work.

In such circumstances, this behaviour is entirely normal and generally is no cause for concern.

The iPad's internal battery charging circuitry generates heat, more so while charging to ~80% of maximum capacity - at which point the charging rate is automatically reduced, with a corresponding drop in heat generation.

While using the iPad, the screen backlight also generates heat, high screen brightness generating substantially more heat than lower screen brightness; CPU/GPU intensive Apps will also substantially add to the thermal load.

HerenaJune has already linked to Apple's support page that describes your iPad's thermal management - this being designed to regulate heat generation to safe levels.

It could be the thermal issue. iPad might becomes overheating when using and charging at the same time. You might also see "Charging On Hold: Charging will resume when iPad returns to normal temperature." in Notification Centre.

If your iPhone or iPad gets too hot or too cold - Apple Support