I'm far from a relationship expert, but the rational people use to call Tim a "bad boyfriend" is completely alien to me.
"He misses dates all the time and even when he shows up he leaves early or falls asleep!"
Yeah, and if you date a firefighter, EMT, ER surgeon or any other form of emergency services worker, they're gonna do the same thing. They will miss dates and need to leave early because they're on-call, and they'll fall asleep because they just got off an 18-hour shift but still wanted to see you.
If that unreliability is a deal-breaker for you, fine, you don't have to put up with it. But that means you shouldn't date firefighters. Or superheroes.
Also, by the nature of the medium, we're only shown plot-relevant dates, and dates are only plot-relevant when they're interrupted. We don't get to see the successful, peaceful dates they're presumably having off-panel for the same reason we don't get to see Tim sitting through his normal, uneventful alegbra lessons.
"But he doesn't tell his civilian girlfriends that so they don't know what's going on!"
And that's bad, yeah. But it's also a staple of the genre, so unless you're blanket criticizing every hero who's ever had a civilian love interest going back to 1905 and the Scarlet fucking Pimpernel, it's not fair to lay the accusation personally on Tim.
Also, that's a big-ass secret to expect a guy to drop for a high school relationship that almost certainly isn't going to last. Because they're in high school.
"But that means he's keeping secrets!"
So?? Everybody keeps secrets. If you think your partner isn't allowed to have secrets in your relationship, you are the problem. That's a weird and invasive belief to have! If the secret isn't about your joint finances, dependants, property, health and/or relationship status (ie, affairs or previous marriages), it's really none of your business.
I've never understood this in fiction, where people act like the very act of keeping secrets is equivalent to lying to your face. Everyone has secrets. Everyone has a right to privacy, even in a relationship. If someone shares their secrets with you, it's a sign of trust and you should value that. But you're not entitled to them, you never will be, and when you get upset with the person for keeping those personal secrets, you're just proving why you weren't safe to tell in the first place.
And that all feels like it goes double when learning his secret would technically make you an accessory to his illegal vigilante activities.
Having friends who are also girls isn't cheating. Getting flirted with isn't cheating. Being kissed by other girls against his will isn't cheating. Realizing that he wants to date someone else and breaking up with his current partner to do so isn't cheating, and it's weird that people act like it is.
You know what is cheating, though? Actively pursuing one boy -- as in, insisting that your random meet-ups are actually dates and either forcing kisses on him or tricking him into kissing you and then mocking him when he tells you to stop -- while you are also, actively, dating and sleeping with another boy.
And yet, the same people who call Tim a cheater and a bad boyfriend freak out at the very suggestion that Steph is a cheater and a bad girlfriend. Double standard much? It would legitimately bother me less if people could admit they both fucked up in typical teenager ways instead of laying the blame entirely on Tim.
More than every other 90s comic in existence? No? So why are you slapping that label on one character? Because then you'd have to admit that [fill in favorite character here] was also sexist at the time, if not still sexist now?
"You have to be able to accept the flaws of your favs..."
Cool, so you're also willing to accept that Steph is a bad girlfriend? That she cheated on her first boyfriend to pursue a sexy superhero, refused to take no for an answer, sexistly insulted and demeaned other women, jealousy jumped to false cheating accusations, violated the one boundary her boyfriend set in their relationship, and beat her ex for no reason?
No??? That all "doesn't count" because "they deserved it"??