MacBook Pro being charged by external monitor; is this okay for battery health?

You can leave the computer plugged in charging when convenient to do so. Neither your computer nor battery will be damaged. An advantage of leaving it plugged in is that the rate of accumulation of battery cycles will be reduced.

An external power supply that provides "USB Power Delivery" (like certain displays)  can not 'force itself' on your Mac. The Voltage and Current are delivered only after your Mac requests and the charger agrees to supply power under certain controlled conditions. The computer is in control of the entire process.

In general, you should ALWAYS connect AC power when it is possible to do so, and only run on batteries (which will be somewhat slower) when no AC sources are at hand. There are three micro-controllers cooperating on battery and charging issues, and your Mac will NEVER over-charge.