Closing an account that you have held for a while can cause a dip in your score for a period. Lenders like to see long financial relationships. This dip isnt because you opened a Monzo account as such. It would be the same if you opened an account with anyone else and closed your HSBC account. You can see your score starting to increase again by the next month.
Your score is determined by the CRA not lenders. Yes a higher score is an indication of your likelihood of obtaining credit (based on models created by the CRAs) but your credit history is far more important (such as defaults, missed payments etc).