Should I clean my boiler room duct (ventilation)?
I think you have a good handle on it.
&quot;If correct, is it a big issue if it is blocked/clogged?&quot;
Yes and no.   If blocked, air will be drawn into the house through chimneys, bathroom/kitchen/dryer vents, leaky windows, etc.  That air would be cold and uncomfortable.   But not dangerous.  It will eventually move to the furnace room.
Clean that screen and the air drawn in will be used by the furnace and water heater and not cool down the house.