Mello was not willing to work with other people (and by other people I mean Near) when L died, but Near was showing how open minded he was, which in this investigation is important. and when Near took over for L, Mello turned to criminals for help, and kidnapped an innocent girl. it was brilliant, but not neccessary.
but if you'll notice, in the end Near pointed out that neither of them would have been able to solve the investigation without the other.
Neither the person who should have been his successor died and was in prison in califora his name is B witch stood for Backup or Beyond Birthday out of anyone he was the closest to L other than Watari he looked more like L other than the Interesting point he was born with Shimigami eyes so they are red and he acted more like him to other than the going crazy and wanting to be better than L and killing people that where allready going to die that day so he just killed them more violently and making this huge case for L to solve but he didt solve it on his own and in some sorta sence B won only a really sorta kinda way because he his juged the person working for L and Failed horridly on the last part of his plan but w.e he still is smarter than near and mellow both he was just a bit jelouse that he was a backup