Does matched betting affect my credit score?

Your credit score is not directly related to matched betting, or any gambling activities for that matter.

The only indirect exception to this would be taking out a loan with which to conduct matched betting. This is highly advised against. You can build your bankroll starting with very little, just through taking signup offers with bookies and converting them for profit. This can be put back into taking more signup offers and repeat. By avoiding taking any credit there is no need for a credit check to be conducted, or your score to be affected in any way. Bookies have been legally barred from offering lines of credit since 2019 and thus have no reason to conduct a check on you themselves.

You can see a list of factors that can affect your credit score here.