It really isn’t an issue. Kimchi was made for hundreds of years without airtight containers. Historically they were fermented in giant earthenware pots with pretty loose fitting lids. Not only that you open it up every time you take some out to eat. When I make kimchi that jar is getting open for consumption basically every day.
Kimchi can get bitter but I’m not really sure why. Most Koreans blame it on bad quality salt but I’m calling BS on that. I’ve also heard that too much ginger can make it bitter, but it’s a mystery to me.
But that’s after fermentation is either complete or well under way. You’re still risking contamination if you expose anaerobic bacteria to air. Give the bugs optimal conditions and they will produce the best flavor, texture, and have more longevity.
Some people are comfortable scraping mold (which actually grows roots deep into the ferment if it gets big enough) or kahm yeast (harmless but detracts from flavor) off their ferments, or don’t mind that their ferments spoil more quickly than they would if they had been fermented without exposure to air. But as a best practice, no, I wouldn’t recommend it.
It’s not after fermentation is complete or under way, I’m speaking only from personal experience (I won’t claim to be a fermentation expert) but I like fresh kimchi so I open mine up every day from the day I make it to take out kimchi for daily consumption. If you’ve salted your kimchi properly you won’t get mold that easily. Not saying it’s best practice or not, but properly salted kimchi is pretty resistant.