See direct quotes from his negative comments about Daxam here:
Episode 2x04: Alura's hologram: "Daxam. Sister world to Krypton. A race of bullies and hedonists, committed only to their own pleasure. They are selfish...". Kara: "Bye, mom! Sorry about that." Mon-El: "No, it was, uh, an apt description."
Episode 2x06: Mon-El: "Yeah, you never met my parents. They weren't exactly role models." Kara: "But mine saw the end of the world coming, and didn't do anything about it." Mon-El: "Hey, it's not It's not your parents' fault. It's not even Krypton's fault. Okay, we shared a star, that's all."
Episode 2x09: It was the first mention that Daxam had slavery. Mon-El: "Wait, this is Maaldoria?". Kara: "You know where we are?" Mon-El: "The locals call it Slavers' Moon. The royal family on Daxam used to buy their slaves here." Kara: "I forgot. They had slavery on Daxam". Mon-El: "There were a lot of things there I didn't agree with".
Episode 2x09: Mon-El said "There wasn't much to inspire me on Daxam". Kara: "What about the Prince?". Mon-El: "He wasn't worth admiring. But I wanna be". (basically admitting that HE was not worth admiring but wanted to).
Episode 2x14: Kara: "If your father walked through that door right now, after all these years that you thought he was dead, what would you do?". Mon-El: "I would run." Kara: "Run?" Mon-El: "Because he was not a good man."
Episode 2x16 is when Kara meets his parents. This conversation is during the meal at the spaceship: Rhea: "And to find you. So that we might return, now that the atmosphere is hospitable. To make Daxam great again." Mon-El: "Daxam was never great."
Episode 2x16 (break up scene): Mon-El: "I'm the former Prince of Daxam. And I was a spoiled, useless person, but I didn't know. Until I met you." (I believe that this is literal - it never occurred to him that his way of living was wrong until he came to Earth).
Episode 2x16 (no witnesses, just Mon-El and his parents at the spaceship after Kara broke up with him): Mon-El: "You never cared about our people before." Rhea: "Things will be different this time." Mon-El: "They won't. It'll be more of the same. I think about our lives there and it makes me ill. I detest who I was. I was ignorant, and blind to those around me and letting myself be propped up at the expense of our people. And I'm tired of being served and I'm tired of taking the easy way out. And I'm done being your prince." Rhea: "You sound just like her. That Kryptonian girl has poisoned you". Mon-El: "No! Mother, that woman is the best thing that's ever happened to me. The best thing I know. And whether we're together or not, being near her, it makes me a better person. It makes me the person that I want to be." (note that he wanted to be a better person independently of getting her back or not).
Episode 2x17 (at the spaceship, after he agrees to return with them to save Kara from Rhea): Lar Gand: "Your life will always be Rhea's responsibility. But son, you belong with your people. It means so much to them to have you back." Mon-El: "If this is about the people, then some things have to change. One day I will become king. And I want to be a different kind of king. And why not start now? Why not rebuild our kingdom better than it was. And make justice available for everyone. And get rid of rank and make it so that everyone's voice is heard equally." Lar Gand: "This is absurd." Mon-El: "Why? Why is that so absurd? We have a chance to start again, Father. So why should we go and just recreate the same broken system as before?" Lar Gand: "Because it's what the people need. Do you think we survived the destruction of our world by giving everyone a say? Our royal house stood for a thousand years because the people need our order. And our leadership." Mon-El: "Our house would have fallen. So rebuild with me, Father. Please." Rhea "Enough! (she slaps him) It will take us four years to reach Daxam. Maybe if you spend that time in a cell you'll be cured of these misconceptions." (she calls the guards and he is taken prisoner). Again, this conversation happens with only guards as witnesses, with no hope of returning to Earth or getting Kara back. He just said it because it was what he thought.
I don't think that Mon-El started to think about how bad was Daxam society until he decided to stand up against Parasite (this happened at the end of 2x07) and started to question what type of person he could be.
Note that Mon-El woke up at the end of 2x02 and 2x03 happened in a couple of days. 2x04 is the chapter when Winn and Mon-El go to an bar with a Halloween party (meaning that this happened on 30 October 2016 ). So he woke up from his coma roughly 5 months before his identity was discovered (yes, I know that Kara said in the Flash crossover that he had lied for 9 months, but that would roughly be the moment when his pod landed in Earth - writers in the Flarrowverse are never good with dates e.g. how many years since Kara landed in Earth? they have given at least 3 different dates). The Halloween reference gives us an idea of when 2x04 happened and the rest of episodes (as usual in the Flarrowverse) are supposed to happen roughly when they are released (e.g. the Mxyzpltk episode with Valentine's Day was released on 20 February).
Remember that TPTB have said that they wanted this character evolution to start from the most distant point to being a hero (shallow and selfish) to thoughtful and selfless. As he woke up roughly 5 months before his identity was discovered, I find the level of distaste that he shows in 2x16 about Daxamite customs to be a quick evolution, to be honest. I don't think that it would be realistic that he started to be self-aware before 2x09.
Also, as a secondary character and particularly a love interest, his POV does not get a lot of time (just the same as with any conflict with Iris or Felicity what the narrative wants us to look at are Barry's and Oliver's POV).