When a lender pulls your credit score and report as part of a loan application, the inquiry can cause a minor drop in your credit score (usually less than five points). This shouldn't be a concern, though, as the effect is small and temporary, and on its own shouldn't cause significant damage to your credit score or affect a lender's decision. In addition, credit scoring models recognize rate shopping for a loan as a positive financial move, and typically regard multiple inquiries in a limited time period as just one event.
That said, this is not the time to apply for credit you don't strictly need, such as new credit cards or a student loan refinance. Save those applications for later, after the mortgage loan has closed and the house is yours.
If you aren't submitting a formal loan application yet but want to get prequalified so you'll know how much house you can afford, your lender will likely base its prequalification on a "soft" inquiry. This type of inquiry does not affect your credit scores.
Once you've been approved for a mortgage and your loan closes, your credit score may dip again. Good news: Since you've already been approved for your home loan, this temporary drop may not matter much.
Why does your score drop when you get a new mortgage? Your mortgage is a big loan and it's brand new. Credit scoring models don't have evidence yet to show you'll be successful at making your payments on time. A new account also lowers the average age of your accounts, a factor that accounts for a small part of your credit score. This temporary drop in your credit score should begin to resolve after a few months of paying your loan on time, all other things being equal.