The President of a Washington consulting firm said yesterday it was "an awful mistake" to suggest it had an influential relationship with Sen. Abraham A. Ribicoff (D-Conn) in connection with a tax relief bill Ribicoff is sponsoring.

Harold B. Malmgren, president of Malmgren Inc., said his firm sent out only a few copies of a letter in mid-to-late January offering to represent for $200,000 a small number of corporate clients interest in the issue - the taxation of Americans broad.

The letter emphasized Malmgren's former position as an aide to Ribicoff and that of Jeffrey Salzman, another consultant for Malmgren Inc., who was legislative assistant to Ribicoff until last November. Salzman helped draft the bill in question.

Ribicoff, a senior member of the Senate Finance Committee who has been seeking to restore some of the tax benefits American abroad enjoy prior to the tax Reform Act of 1976, said he was shocked" by the letter. In it, the Malmgren firm claimed to be "in an usually good position to influence the outcome of the debate" on the issue "and move it in a good direction."

"It was just bad judgement," Malmgren said, adding that nothing came of the proposal.

A former deputy special trade representative under president Nixon and Ford, Malmgren said his economic consultant firm, in business for about 18 months, specialized more in the field of international trade and finance. It clients include the Japan Whaling Commission, which hired it recently for six-month fee of $112, 500 to lobby for a "sensible whale-hunting quota.