A fair warning that I really dislike the proliferation of addenda (IMO, they are whiny, and 95% of them make the author sound entitled and obnoxious), so take this advice with a grain of salt.
I think addenda should only be used for circumstances outside of your control. For example - if you came down with mono in the middle of the semester, if you suddenly had to become a caretaker for a grandparent or child, if you lost your scholarship and had to work fulltime to support yourself, if someone close to you died on the morning of the LSAT, etc.
Adjusting to college isn't really addendum worthy. I mean, what are you going to say that will really enhance your application? Many freshmen have a hard time adjusting; the circumstance isn't novel, nor is it something that was really earthshattering enough to seriously justify lower grades. It is what it is. There's nothing to explain.
i was very much of the same opinion as you, but then i read Anna ivey's book and she says difficult-adjustment-college-omg (lol, scratch the 'omg') addenda are okay. in fact, i think she says they're good. granted anna ivey isn't the end all be all of the admissions process, but i figure her word's pretty good? idk i (and she) could be dead wrong though.
the strange thing with me is that when i hear ppl say they had mono or something, i always feel a bit suspicious. i guess its the cynic in me. but i feel like Adcomms *must* feel a bit suspicious about all the mono-addenda they read.. no? idk, what are your opinions?
Probably depends on how severe the mono is. I had mono in my freshman winter but I could still go to class and got good grades.