The average yield on one-year Treasury bills declined again yesterday while the yield on money market mutual funds continued to rise.

The return on the bills sold at auction was 12.509 percent, down from 13.180 percent a month ago. It was also the lowest yield since Dec. 23.

The rate on All Savers certificates will dip next Monday to 10.16 percent from the current 10.79 percent. This yield equals 70 percent of the average annual investment yield on one-year Treasury bills, or 14.52 at yesterday's auction.

All Savers certificates, which first were offered Oct. 1 of last year, will be sold until Dec. 31, 1982. Individuals are allowed to exempt from their income taxes $1,000 in interest earned on these certificates; married couples filing jointly can exempt $2,000. This is a one-time, not an annual, exemption. At 10.16 percent, a saver has to deposit $9,842.50 to earn $1,000 interest. At 10.79 percent, the required amount is $9,268. For couples these figures should be doubled.

Meanwhile, the average seven-day yield on money market funds rose to 13.52 percent from 13.45 percent, according to Donoghue's Money Fund Report of Holliston, Mass. The Investment Company Institute reported yesterday that sales of money market funds spurted to a record $189.6 billion during the week ending Wednesday.

This is the fourth consecutive week of total increases after several weeks of net withdrawals.