The way DeLillo introduces Marguerite at the very beginning makes her seem like a relatively sympathetic mother in a bad situation, but still trying her best. This is right after the scene of Lee "playing hooky" on the trains, and he and Marguerite are sitting together watching TV. Lee is telling her about riding the trains, and Marguerite notes that it's okay to skip a day of school or two since the other kids bother Lee and he's likely to have "turbulence running through him," as a boy without a father.
"She sat and listened to the boy's complaints. She couldn't fry him a platter of chops any time he wanted but she wasn't tight with the lunch money and even gave him extra for a funnybook or subway ride" (4)
Hearing about Marguerite's multiple failed marriages and her low jobs also add on to make her seem like a mother who's also gone through some hard times but is still trying to stay supportive of Lee where she can.
Except that under the facade, Marguerite's character has some ambiguous quirks and weird flaws that aren't much highlighted in the book so far--but may be the reason for some of Lee's antagonism (in addition to just his asshole character):
Although we get some positive images of Marguerite's mothering, we also later learn that she sometimes leaves Lee in the orphanage for periods of time. The reason isn't given, but it seems rather strange to casually do this multiple times; it doesn't seem that they are quite destitute enough either, for a mother to leave her child this way.
In addition, Marguerite doesn't seem to really try and get Lee back in school for his benefit after receiving official notice of his skips. She just complains for "two hours in [a] high piping tone," not an effective or conscientious way or solving the problem (6). The long sections of Marguerite's rambling narrations to the ambiguous "your honor" are a similarly ranty, expressive, and rather untrustworthy side of the woman. In one single section she bemoans her family situation and wavers between unrelated (and probably untrue) assertions such as Lee buying her a parakeet with a planter and ivy and food etc. (48).
In class, Mr. Mitchell mentioned that Marguerite in the records is known to be a pretty weird person and tried to get off as much publicity as possible from Lee Harvey Oswald. She seems to a rather ambiguous character so far in Libra, but is definitely already showing her weird side--whatever future development comes to play, I'm on the side of taking everything Marguerite represents with a grain of salt.