They ate a much broader range of fruits and vegetables than just figs:

• Leafy greens included cabbages, mustard greens, lettuces, mallow, and various wild greens such as nettles and dandelions.

• Root vegetables included radishes, turnips, parnips, carrots (and the leaves of these could be eaten as greens)

• Other vegetables included onions of various types (very common), and garlic, and cucumbers.

• Legumes were an important part of the diet, most notably lentils. Other legumes included broad/fava beans, chickpeas, lupins, and peas.

• While figs were an important fruit, other fruits such as grapes, pomegranates, and olives were common.

• Nuts such as pine nuts, pistachios, walnuts, almonds, chestnuts, and others, were eaten.

• Cookings oils included butter and various animal fats as well as the ubiquitous olive oil.

Some of these foods were seasonal, but could be preserved and thus available for much of the year.

What reason is there to think that malnutrition would have been a problem, as long as sufficient quantities of these foods were available (and affordable)? Overall, their average diet depended much more heavily on grain than typical modern European diets, with barley and wheat breads and porridges providing a large part of the calories. For the poor - especially the urban poor who would have had less opportunity to collect wild greens, and could afford to buy less variety of food, the diet could be somewhat low in protein, but the heavy use of legumes would have avoided this problem for most. The typical diet would likely meet the USDA 2015-2020 diet guidelines, with a higher proportion of carbohydrate, less sugar, less fat, and sufficient protein (but possibly at the low end of the scale for the poor). Rather than malnutritions such as vitamin deficiencies, bigger risks would have been famines and/or poverty leading to insufficient calories.

On average, less need for multi-vitamin tablets than we have today (and a good thing too, considering their unavailability at the time). Perhaps the main thing in their diet that would be disapproved of by modern authorities would be the amount of wine drunk. The wine was usually drunk diluted, so unless a large quantity was consumed at a serious drinking party, the amount per meal could be a modest one glass by today's standards.

Afterword:

But the serious drinking parties could be pretty full-on. The description that the poet Eubulus attributed to Dionysos, the god of wine:

Three kraters only do I mix for the temperate - one to health, which they empty first, the second to love and pleasure, the third to sleep. When this is drunk up the wise guests go home. The fourth bowl is ours no longer, but belongs to hubris; the fifth to yelling, the sixth to prancing about, the seventh to black eyes. The eighth brings the police, the ninth vomiting, the tenth insanity and hurling the furniture.

makes clear the possible consequences. Even the "wise guest", leaving after the third krater (shared bowl) might have had a litre of wine - diluted perhaps 50/50, and equivalent to about 2/3 of a modern bottle before dilution - still a non-trivial amount.

Reference:

U.S. Department of Health and Human Services and U.S. Department of Agriculture. 2015–2020 Dietary Guidelines for Americans. 8th Edition. December 2015. Available at http://health.gov/dietaryguidelines/2015/guidelines/.