It's not something I bother doing, but I think the generalized way those horoscopes are presented can just be a matter of a little bit of fun for many people - kind of like reading the "fortune" in fortune cookies (at least for those who don't put a lot of stock in either). Sometimes they're "neutral" and/or meaningless. Sometimes one seems to hit a chord in some way. (In the case of fortune cookies, sometimes they're just really pointless and stupid - in which case, people can get a little chuckle about how stupid they are).
People who really believe in astrology don't put a lot of stock in those daily horoscopes, People who either aren't sure or else just don't believe just kind of get a little kick out of reading them.
I pretty much think the "mental flaw" is with the person who knows that others read their daily horoscope, reads too much into how much, or whether, they take it seriously; and then jumps to a conclusion of "mental weakness".
There's no mental weakness in simply reading a horoscope because it's there and because it's kind of a 30-second (or whatever) break from the day. If you think about it, though, neither is there "mental weakness" in the person who knows (particularly first-hand) that so many things in life truly ARE a matter of luck, who hasn't seemed to run into much of anything that looks like "good luck" to him, and who reads that horoscope in the hope of, maybe, seeing some words there that (even if the person doesn't really even take them all that seriously) that could even remotely be taken a faint, faint, glimmer of a change in luck (or "luck", depending on whether it's real lack of luck or not).
One sign of at least one personality disorder is being humorless and/or being unable to lighten up when it comes to some things in life (like someone else enjoying something that doesn't happen to interest us very much, or at all. People need to leave the assessment of "mental weakness" up to the psychiatrists. And as for daily/weekly horoscope-reading, to each his own.