There will be battery voltage drop with any mechanical mod. The voltage comes directly from the battery, and as the battery drains during use the vape quality will gradually drop, too.
A "regulated" mod does exactly that. It
regulates the voltage output to stay the same according to your settings, by artificially boosting the battery output while the battery drains via a processor chip.
Some mech mods will accept a Kick device. This is a drop-in module that sits on top of the battery, and will regulate the battery output like the processor in a regulated mod. Keep in mind the Kick takes up room in the battery area of the mod, so you may need to drop down in size of the battery. Some mech mods have battery extension sleeves or caps in order to use a Kick with an 18650 battery.