Maryland Cup Corp., the Owings Mills firm that makes Sweetheart cups and Eat-It-All cones, managed to increase its profits in the second quarter although its sales declined slightly.

The firm earned $5.7 million (55 cents), compared with $4.5 million (44 cents) last year. Revenues were $149.1 million, compared with $150.3 million in 1982.

For the half, Maryland Cup earned $9.5 million (92 cents), compared with $8.5 million (83 cents). Sales decreased to $281.7 million from $286.2 million.

Per-share earnings for 1982 reflect a 3-for-2 stock split this January.

"Our ability to surpass last year's profits, under difficult economic conditions, is due primarily to better control of inventories, costs, selling and other expenses," said President Samuel Shapiro. Maryland Cup is the nation's largest manufacturer of paper and plastic food service products.

Another Maryland firm, Monumental Corp. of Baltimore, said yesterday its earnings were down in the first quarter to $3.2 million (46 cents), compared with $3.4 million (50 cents) in 1982.

Leslie B. Disharoon, chairman of the life insurance company, said the first-quarter figures were expected, and results for the rest of the year should be better. Individual life insurance sales jumped sharply in the first quarter, he said.

The Washington Corp., a Bethesda real estate ownership firm, made a first-quarter turnaround from last year, posting earnings of $130,201 (5 cents).

In the first quarter of 1982, Washington Corp. had a loss of $179,198. Gross income jumped from $281,158 last year to $497,723 in the period ended March 31.

The company said it realized extraordinary gains from the early retirement of $250,00 of 6.5 percent convertible debentures and from selling a piece of land in Prince William County, Va.