Are there any MOSs that are mocked for being 'unimportant'?
Most assuredly! The most well-known example of a job that gets thoroughly mocked is 42A, Human Resource Specialist. When someone wants to insult someone for incompetence, “Looks like you should’ve been a 42A.” If someone is very bookish and having trouble with the physical part of their job, “You didn’t enlist as 42A, you know.”
Of course, these taunts are all rather silly when you realize 42As are often the ones responsible for us getting pay, BAH, BAS, leave, and so on.
It’s probably easier for me to list the occupations/types of units who AREN’T mocked for being unimportant.
Aaaaand that’s about it. If any given soldier says they’re any one of these things, most people will probably go “Okay, you’re legit.” Not all of these things are mutually exclusive of course; some of them add on additional badassery. 91B is Wheeled Vehicle Mechanic, and they can be attached to almost any unit, but a 91B stationed with infantry is probably thought of with more esteem than a 91B stationed with an Air Defense unit.
Pretty much everyone else is fair game for being made fun of. Air Defense, (my branch) cook, any and all office jobs including S-1 through S-6, and gosh pretty much anyone and everyone who isn’t doing physical bullshit 24-7.
Of course every MOS in the army is important, sure, we say that, but sometimes it really does get to you and you don’t feel that important. Feeling like my job was worthless was a big reason I didn’t stay in as long as I intended.