Are there "restart" mechanisms for candidates seeking a graduate admission with a mediocre undergraduate record?
I don't think it is possible to give a general answer to this question. There are too many variables. On one hand people do recognize growth in a student and that new work may be more important than old. However, it also depends on what you mean by a "certificate program". There is a lot of variability in that. Some have good reputations, others not. How does a grade in a given certificate program translate to a grade in a university course? How much skill was gained in the course. 

Yet another variable is which courses were taken. Is it more or less the equivalent of an undergraduate major, including all of the usual courses. 

Given all of these variables, the only answer that is valid will come from a particular institution evaluating the candidates complete record and their statements about themselves and their goals. For some, the answer would be no, for others, possibly yes. The only way to know is to ask and the best way to ask is to actually apply and supply all of the needed information, including reasons why the record is a bit "unusual."