It's evidently going to take more than one bad year in the financial markets to dull investors' voracious appetite for mutual funds.

Thanks to such events as the crisis in the Persian Gulf and the onset of an economic slump, just about all types of long-term funds have turned in poor performances for 1990.All 14 categories of stock and bond funds monitored by the Wiesenberger Investment Companies Service showed declines in their average net asset values for the 12 months through Nov. 30.

Yet sales of new fund shares to investors show no sign of flagging, setting their strongest pace since the most ebullient days of the 1980s bull market.

Through the first 10 months of this year, reports the Investment Company Institute, the industry's main trade group, sales of long-term funds (as opposed to money-market and short-term municipal bond funds) came to $125.18 billion.

That put them on the verge of surpassing the $125.71 billion total recorded for all of 1989.

When full-year figures are in, they appear certain to rank as the third best ever, behind only 1986, with sales of $215.85 billion, and 1987, with $190.63 billion.

An especially striking element of this year's activity is that sales of stock funds have been running slightly ahead of bond and other income funds, which represent a larger category.

That has happened even though the average equity fund's net asset value dropped 10.7 percent over the last 12 months, by Wiesenberger's calculation.

Indeed, "not since 1974 have such a preponderance of equity mutual funds shown a loss in any year," observes Reg Green, editor and publisher of the Mutual Fund News Service in Bodega Bay, Calif.

Steve Norwitz, a spokesman for the Baltimore fund-management firm of T. Rowe Price Associates, points out that business has been notably hot for international equity funds, even though nearly all overseas stock markets also have fallen this year.

The ICI numbers show international fund sales of $6.97 billion through the first 10 months this year, more than four times the $1.54 billion tally at the correspond-ing point in 1989.

What to make of all this bullishness in the face of adversity? Most analysts say it sends a clear-cut message that fund investors are seizing on what they perceive as a buying opportunity.

The memory is still fresh in their minds, analysts reason, of the profits that awaited most people who stepped up to buy in the aftermath of the crash in '87.

Some skeptics see this is as a danger sign. Typically, they say, bear markets don't bottom out until mutual-fund investors have been scared away in large numbers.

Optimistic observers, by contrast, say it could be a sign that fund investors have learned to think longer-term, building up their holdings in good times and bad.

In either case, keen enthusiasm among the public for funds may be at least partly offset by a wary mood among fund managers themselves. At the end of October, stock funds that report to the ICI were keeping a record 12.9 percent of their assets in reserve, "parked" in money-market investments.