The use of credit cards is becoming increasingly common in Nicaragua; most mid-range to high-end restaurants and shops in major tourist areas will accept some form of payment by plastic.
If you're traveling to Nicaragua, though, you should in no way rely on credit cards to provide for all your needs, especially if you intend on leaving the major cities. Many places will only accept cash. The power also cuts out frequently, especially in the Caribbean, and will render your cards useless, since the card scanners will no longer work. The general rule is: the smaller and more remote the establishment, the less likely they are to accept payment by card.
Visa and MasterCard are the most widely accepted cards and carrying both is a good idea as many places will either accept one or the other. (American Express and Dinners Club are also sometimes accepted, but not as frequently as Visa and MasterCard.)
NOTE: Before you leave home, make sure to inform your bank of your travel plans to ensure that your card is not blocked while you are in Nicaragua. As a precaution, take photocopies of your cards (leave a copy with a trusted friend at home and keep a copy with you, but separate from the actual card) and keep the number of your credit card company handy, so that you can easily call up and cancel your cards if they are lost or stolen.