I think that Mary the potential to be a great queen, if her parent's divorce had not crashed in upon her in her sensitive teenage years, she who had been the cherished only child until then. Yes, Elizabeth had a troubled childhhod, too, but her troubles started at age three, which made her tough. Mary grew up as the heir to the throne, and suddenly became an outcast, separated from her beloved mother Queen Catherine. Helen_A pointed out that Mary's mother and grandmother (Isabella of Castile) were both too black and white about everything (rigid?) and that Mary was just like them (true) which would have prevented her from being as great a queen as Elizabeth, whether or not her parents divorced.
I do think, however, that the divorce made Mary more extreme, and more neurotic, than she otherwise might have been. Also, many rulers in those days, even great rulers like Isabella, were rather black and white, "rigid" as we would say today, in matters pertaining to religion. Both Henry VIII and Elizabeth had people executed for having religious beliefs different from their own, although Elizabeth did not have as many killed as Mary did. But Mary had lost her sense of proportion.
I still maintain my opinion that Mary initially had the potential to be a good ruler if her world had not been turned upside down. But we will never know for certain.