Only way this would make any sense is if the interview is witnessed and fully recorded - if so, then awesome.
Just like a prosecutor would do with a local perp. Always glad to talk to perps without their lawyers, get them on record, find inconsistencies and lies.
Anything short of that is yet more corruption.
Well, Mueller is assigned the job of picking up and completing the investigation that Comey described during a Congressional hearing. for short, I refer to that investigation as "Comey's investigation," even though he is only the messenger as to its existence, to the public, via Congressional hearing.
Mueller wants to pick up that investigation, and no more than that investigation. It's in Mueller's interest to narrow that scope as much as possible.
Comey is no way relevant, let alone "of interest" to the investigation of collusion between the Trump campaign and the Russian government