As others have stated that's what these are for.
https://www.overtons.com/boat-vent-i...nt-314933.html

Vents help to keep the mold down. If you live where mold is an issue, buy a bag of charcoal, lay it flat on a thick trash bag, and cut a big letter I, open the cut portion and fold back. The bag of coal will absorb the moisture. Old trick we used when stored in caves in the KC area. Learned that the hard way. My first boat I stored in there, without the bag of coal, had wild mushrooms all over the back seat, took me two days to clean that mess up and disinfect.

I've got one cover that utilizes the type of rod #2 and one that uses type #3 and the #2 is definitely better IMO. Both have heavily reinforced areas that they engage. Type #2 hasn't been prone to letting water in as the fit is tight and even if gusty the support rod holds its position.
The type #3 method simply pushes up against a heavily reinforced area on the cover without any holes to catch so it's definitely not going to let any water through but no matter how much tension I get on the cover with it it seems that the wind can occasionally cause the cover to whip up and down and then the support rod falls to the floor resulting in a very bowed cover which might not be discovered for a long time.
Never used the snap type #4 but I would think that it's not as convenient in terms of how to unbutton it when on the outside wanting to get the cover off .
One cover is made of Sunbrella so it's naturally slightly breathable the other is stamoid and has 2 clamshell breathers installed. They help with ventilation but can let dirt and grime in when the wind is coming from the wrong direction .

Check out the Boat Vent 2 they require a circle cut out and the supporting pole slides into the hard plastic vent. I sewed a second layer of fabric just as a backup. I think they are about $10 and they screw together sandwiches the fabric between the vent and a locking ring.

I have these in my covers and they work awesome. Never had a moisture issue or mildew inside of boat either when I stored outside for long periods before I built my barn addition to fit boat.

I got fed up with the flimsy poles etc so I made my own and I love it. I got a plastic Christmas tree stand and put a piece of PVC pipe in it, I think about 5" dia or so. Cut to the correct length and put the rounded PVC cap on it. Super sturdy, doesn't tip over, doesn't scratch the boat when being moved in/out. I actually made 2 of these, one for the cockpit, 1 for the open bow.

On my Cig I had the cover maker sew in a condom type of fabric about 8" long for the pole to slip into and reinforce well. Never came out while trailering.
Boat stayed in WH when not on lift, so Mold was not an issue. Cover in dock was hung from structure.