Similarly to how a good engineer can be a force multiplier for a team and a bad one can tank the entire team, a good manager can also be a force multiplier for the company, team, and for you individually. And a bad manager, at best, will let you tread water.
If you have ambition and want to grow and learn, you should seek out another manager. (Note what I did not say: you do not have to seek out another _job_.)
A good manager will identify your career goals and give you a framework and guardrails in which you can launch yourself to the next level. A good manager will care about you personally ("nice") while being tough about the realities of the business ("serious"). When I hear about a manager that's nice but incompetent, I think of the term "ruinous empathy," which basically means they're so nice that they can't be effective.
Sometimes tough love is what people need, and even if it's not "nice," it's kind and it's the right thing to do.
You have, in my mind, a few options:
1. Be compassionately candid with your boss. "Look, I know your job is tough and that you're a really nice guy and care about us a lot, but if you let the little things slide, nobody will take you seriously, and I want to be taken seriously." If that doesn't work, escalate to the boss. Read up on ruinous empathy and be prepared to give examples. "They are too nice" isn't a good critique, but "Their reluctance to enforce discipline is causing problems A, B, and C" is hard to ignore.
2. Find a mentor either externally or within your company. This means finding someone that you respect that can give you candid advice on how to improve in whatever areas you are interested in. This does not have to be your boss. It cold be another manager at your company, another engineering leader, or even an HR person. Alternately, you can find a new job -- although that's not always guaranteed to be better, as other posters have pointed out!
3. You can abdicate responsibility, say "It's not my problem," and put your energy into non-work things. This is a totally valid approach. It's not what _I_ would do, but it's something that is totally fair. Maybe you want to level up your guitar or piano skills, or you want to get really fit. Take that extra energy you have and throw it into your chosen hobby. Spend more time with your family. Sleep better at night. Work isn't everything. It's just a job and a paycheck.
But my big piece of advice: don't settle. Don't settle for mediocrity. Keep pushing to be a better person, a better colleague, a better team player. Practice giving compassionate and candid feedback. Keep an eye out to avoid becoming obnoxiously aggressive or ruinously empathetic (two sides of the compassionately candid spectrum). Identify your goals, whether that's leveling up at work or leveling up in your private life, and go after them with gusto and abandon.
I've been where you are. It's tough. But you sound like the kind of person that can and will do better.
Best of luck to you.