Randomization is fun until it isn't.
The ultimate point of playing a game with your friends is to have fun. Part of the fun is the risk, but at the same time, it's not always fun when pure random chance does something weird. And even when everything is working normally, you have to have a sense of pacing; there's a point where the players are getting bored and the best thing is to move along to the next thing rather than stretch out a scenario just because everyone's rolling badly.
I fudge things at my table all the time. Usually it's about HP -- either offing a monster early or having it hold on after it technically should have died. Usually it's for dramatic reasons -- I can tell the fight is dragging and it's better to end it now than take another four minutes to get those last few HP, or sometimes I want to let a particular character get the kill, or I want a chance to show off a monster's special ability at least once before they just gank it.
I also fudge monster attacks sometimes, usually in the players' favor. I had a fight earlier this year where a sahuagin baron was just rolling ridiculously well. The third time he rolled a crit against the same character, I just decided to skip it and called it a normal hit instead. It left the paladin with 2 hit points instead of laying on the floor, and made for a more dramatic finish.
Sometimes I fudge DCs, allowing somebody to pass a check that they maybe technically failed because failure wasn't interesting or the game was dragging. That's less common, though -- usually task rolls just are what they are -- but I go even further than that sometimes and don't even set a DC before they roll. Somebody wants to do a thing that should take a roll, but which I didn't predict? I don't necessarily set a DC, I just ask for the roll. 60% or 70% of the time, it's either so high or so low that the exact DC wouldn't have mattered, and the rest of the time I can kind of just judge it after the fact. In that case it's in service of keeping the game rolling, I don't need to take the few seconds to figure out a DC when I can just say "Roll it!"
Some people get very upset about fudging, but it's not a trait of a bad DM. Or at least, not inherently. It's a tool, and like most tools, it's how you use it that matters.
You can fudge for good, in service of the story, the fun of the game, or efficiency. You can also fudge for evil, to make the game unwinnable for the players or otherwise screw them over. But in that case, it isn't fudging that makes a bad DM, it's the fact that they're using the game to be cruel to their friends.