How is Munna randomly drawn? Because people generally describe it as a "pink blob"? Well let's break it down and discuss what the thought process that had to go into making it:
Concept: Well before we even start talking on its design we first must talk about its concept. Now Munna and its evolution, Musharna, are notable in BW for having a connection with a major feature in the game, the Dream World. Its explained the Dream World machine runs on the dream smoke from Munna & Musharna. So it could be their existence actually comes from GF deciding they needed an explanation for the Dream World machine and thought a product of a Pokemon should be responsible.
Okay, but how did they from that concept to the Munna family? Well since the theme was "dreams" they probably looked through mythology for a creature relating to dreams settling on the Baku, the Japanese tapir spirit that eat dreams. From there they probably then through around a batch of design ideas which we'll now go into:
Shape: Okay, so they know its a tapir but how much of a tapir should it look like? One thing to remember is that the Drowzee family is too based on a Baku, so they may also want to stay clear having it resemblance. So they decided to go with quadrupedal tapir (opposed to Drowzee's bipedal) that looked more cartoony (opposed to Drowzee's more detailed/realistic look), explaining the simple shape. Also since Munna is the pre-evolution its even more simple, smaller, and stout/compact.
Color: The color was probably picked along with the Dream World's mist color. They probably wanted a color that was bright, eye-catching, and maybe a tad bit unnatural/ethereal looking. Pink fits the bill nicely (yes there are natural things that are a vibrant pink like flowers, but those are exceptions as they're trying to stand out thus such a distinct color).
Facial Features: Keeping it simple they chose small, oval, solid eyes; a darker pink/reddish color to stand out from the rest of the body (and two eyelashes as it was probably decided that outside of story they'd probably best market this Pokemon to girls thus making it look more feminine and cute). Now in addition to a Baku/tapir, the Manna family is also based on an incense burner to go along with it producing Dream Smoke. So they gave it a simple oval "nose" which resembles a hole of an incense burner and also have a place for the smoke to come out of (as seen with Musharna).
Floral Pattern: I can only imagine that there are TONS of little notes around the GF offices about trivial things mentioned in the games they could later use as design inspirations. One of these little notes no doubt remembered a certain Lass in Gen I saying she would like to have a pink Pokemon with a floral pattern on it. Well, they just made a simple pink Pokemon who's looking rather dull. They probably already decided to market it to girls, so why not also add flowers onto its back for good measure and as a callback to that Lass?
Now let me say this is all just a guess how the thought process went. Another thing to keep in mind is that GF might have made Musharna first (since its more of the mascot of the Dream World than Munna, even visibly giving off Dream Smoke), decided it needed a pre-evolution, and so made Munna thus explaining its more simple design; they had to make a mini Musharna who already looked pretty simple.
But still, just from this simple Pokemon I could infer a batch of decisions that might have gone into making it. As
Integer Mova said, the only Pokemon who were made off the cuff with little concept discussion was the Gen I and probably II Pokemon since they were the first few batch. Discussion about their design happened, it just probably wasn't as much or rigorous as a new Pokemon now goes through. Thinking about it, it could be why people generally like the Gen I and II Pokemon more, they have a sort of laissez-faire feel to them while newer ones feel like they are more committee made to look a certain way.
