The current GDP is $27.9 Trillion. The total market cap of US stocks is $42.4 Trillion. So according to the Warren Buffett indicator the market is currently 52% overvalued.
Lower interest rates decrease the cost of capital, which means companies can invest more easily. Also, it pushes investors to invest in higher-risk assets like the stock market as other assets like the bond market have lower returns. Thus, in times with low interest rates, it might justify higher stock market valuations. There are other forces that have an impact on stock market valuations and that are not covered by the Buffett indicator, such as that the US GDP doesn't fully reflect the world economy wheras the stocks included in Wilshire 5000 do include certain international business activities.
The Wilshire 5000 is a stock market index that includes almost every publicly traded stock in the United States.