It is absolutely possible to be a functional methamphetamine user. Methamphetamine is prescribed for ADHD under the brand name Desoxyn. Not as often as amphetamine, largely because of the tighter legal restrictions on methamphetamine, but there are still
plenty of people who take that drug daily without fitting the stereotype of a meth addict (not to mention all of the functional illegal meth users who you wouldn't hear about because why would they want other people to know about their illegal habits?)
Meth does not make you incapable of holding down a job. It will probably make you incapable while you're in your chasing-the-high phase where you're constantly upping the dose to outrun your tolerance. But in the climate of prohibition, everyone either stops doing that at some point or reaches an equilibrium where they can't afford higher doses so they settle into a maintenance dose where they would be totally capable of holding down a job if they hadn't spent the last n months making themselves unemployable.
And why do they become unemployable? Because the word getting out that they use meth in itself makes them unemployable, and then because the price of meth is inflated due to prohibition[1], they turn to crime to make enough money to pay for their addiction.
Legalizing meth would certainly lead some people who wouldn't have otherwise tried it to try it, and some fraction of them would become addicted and suffer the health consequences[2], and many of those who got addicted would in fact become burdens of the state. But life would be better for anyone who did find themselves addicted, life would be better for people who didn't choose to try the drug on account of lower crime[3]. Some people who would try meth under prohibition would be less interested in it because if it were legal it would fail to signal their disregard for authority, and it's possible that the different personality types of new addicts under legalization would lead to different outcome, but it's probably not worth speculating about what differences there would be.
Legalization trivially reduces crime by denying an income stream to organized crime. If public policy under legalization were not entirely incompetent, and we were able to either help a significant number of addicts hold onto a legitimate income, or help a significant number settle into an affordable maintenance dose (which would be easier than it sounds since their drug would likely be much cheaper), then the criminal activity of drug users would likewise plummet. These cost savings surely offset the increased burden on the social safety net.
[1] This is generally the case with illegal drugs. Generic Desoxyn is actually extraordinarily expensive, at 1 USD/mg in the US. which is higher than the street price in some parts of the US per https://havocscope.com/black-market-prices/meth-prices/. It's possible that this has to due with the cost of achieving pharmaceutical-grade purity, but I'm skeptical of that. I think it's more likely that the price of the pharmaceutical drugs reflects one of the many market failures of the US healthcare system, or the costs of DEA licensing for its production and distribution. But it's worth acknowledging that a quick google search doesn't back up my claims about price inflation.
[2] The health consequences of meth are also exaggerated by prohibition. Meth is somewhat neurotoxic for chronic high-dose users, and definitely caridotoxic. The skin issues associate with meth addicts are caused by the meth itself combined with obsessive picking at the face, but while meth can cause some dental problems as a result of clenching the jaw and gnashing one's teeth, it's likely that most of the dental issues associated with meth addicts are actually cause by impurities in the drug that's available to them, a result of prohibition.
[3] This is critical: the consequences of drug use would be redirected from people who made no choice whatsoever to involve themselves in drugs to people who at least made some choice, even if they somehow didn't fully comprehend the consequences of that choice. And even for the latter group, the consequences wouldn't be as bad as they are under prohibition, though that group would probably be larger.