What do you mean by "dirty bulk?" There isn't really an actual thing as a dirty or a clean bulk. If you are talking about what types of food you are eating, well the actual types of food isn't going to make much difference with regards to muscle vs fat gain. By types of food, I mean eating a "dirty" meat like beef versus a "clean" meat like chicken. That doesn't matter. Training with weights will skew that ratio way more than food can. Obviously getting enough protein in the first place is going to be important. Just set your macros and total calories, hit that and you should be fine.
Before we get too carried away, let me just say: YES - clearly some foods are better for overall health than others, but that isn't going to impact how much muscle you gain versus fat. Eat healthy because you don't want to die early, but don't sit there fretting over your boiled chicken breasts thinking that eating bland food is going to make you huge.
Now... Is it reasonable to expect 5 lbs of muscle gain out of 20 lbs of overall weight gain? No, that's not reasonable at all. You should be making much better gains than that. At the very least, you should be getting 50/50. What can cause problems with this? Gaining weight too fast is the biggest culprit. Your body simply cannot create muscle that quickly. So if you pack on 20 lbs in four weeks, then yeah, you'll be lucky if 5 lbs of that is muscle. But if you gain that 20 lbs over 10-12 weeks, then you should expect a much better ratio of muscle gains compared to overall weight gain.
Of course, without access to clinical level tech, there isn't a good way to measure exactly how much of the weight you gained is muscle, so some of this is just theoretical wanking. But you can take measurements of your arms, legs and waist in order to get a good general idea of what sort of gains you are making. If you store a lot of fat in your arms and legs, this might not be as helpful but generally, watching how fast the waist measurement increases can help you to see how much fat you are actually gaining.