No, it isn’t. A 1915 SCOTUS ruling stated that "“carries an imputation of guilt and acceptance of a confession of it.” But that reasoning, according to this analysis is flawed.

The Court’s reasoning is circular. The Court states that a pardon, unlike a grant of legislative immunity, “carries an imputation of guilt,” and, this is advanced as the reason for allowing the pardonee to refuse the immunity tendered by the pardon. A pardon, however, carries an imputation of guilt only if the validity of the pardon depends upon its acceptance by the pardonee. If the requirerent of acceptance is removed, the imputation of guilt vanishes. It becomes circular, therefore, to use the imputation of guilt argument as a justification for the requirement of acceptance.

Indeed, a recent appeals court ruling supports this analyses and states “not every acceptance of a pardon constitutes a confession of guilt.” Presidents can pardon people they believe to be innocent. And one does not need to accept a pardon for it to take effect. If a pardoned person does nothing, then they still receive immunity from prosecution conferred by the pardon without confessing to any crime. Pardons can even be granted posthumously. Obviously a dead person cannot confess to a crime when they are pardoned.

Edit to fix quote.