I take a similar view of both. Like I said, if you want me to overlook the issues with Ukraine's elections, then I consider it only fair to overlook issues with the referendum. Otherwise, I will be skeptical of both. What I won't do is overlook the problems in one case but not in the other, because that would be biased.
Regardless of the numbers in the referendum, it seems clear to me that there is/was some genuine support for it. If there wasn't, then the government wouldn't have banned the party popular in that region. Whether it's 40%, 60%, 80%, whatever, I don't have enough evidence to determine. But, the reason that I don't have enough evidence is because the government of Ukraine refused to give them an official referendum, refused to let them vote for their preferred party, refused to give them a voice in government to express their views in a peaceful and legal way. That is evidence that they themselves believed pro-Russia sentiment was high in those regions.
You don't just need to prove that the DPR is illegitimate, or that the Ukrainian government is marginally more legitimate, because if they are both illegitimate then I'd still want to stay out of it. The Ukrainian government needs to be legitimate enough to be worth fighting for, worth sacrificing and dying for. Nobody should be drafted and forced to fight for a marginally more legitimate, but still illegitimate, government. The human cost isn't worth it.