1) And Sophia failed so Arland is remember more positvely now. Levi Dyrden says the name "Dryden" is looked down upon in Fereldan.
Also she used Demons. That's a big no-no in Andrastarians countries so she get's even less points.
2) Alistair (his own nephew) and one other person saved his family no Warden group showed up to save the day.
For Fereldan, Teagan doesn't know that the Wardens have to give their life to stop the Blight so, to him, Grey Wardens are darkspawn enclopedia's at best and an army of Dwarves, elf/werewolve, mages/templar and fereldan soldiers saved the nation with you and Alistair/Loghain who just happened to be Wardens.
3) That's nice. Now go disband since your duty is done and stop hanging around his country 2 years later.
Loghain also saved his country. You don't see Teagan kissing his ass 30 years later.
1. Wrong. Arland is a barely known king and little is known about him isn't positive or I guess the civil war that followed his death is a sign that he left a good and stable legacy? Even then, I doubt that Levi and his family would stay quiet about what they found out in Soldier's Keep with the intact documents and so-on that were found there, including various letters and items from past banns, arls and so on indicating that whether he was the winner or not, Arland was a madman.
Even then, the Wardens still saved Ferelden in present-day despite being unlawfully declared outlaws.
2. They were still Grey Wardens and were limited in number because of Ostagar and Loghain blocking the Orlesians from sending much needed help in manpower and Grey Warden reinforcements. And numbers don't change the Redcliffe, Teagan's family and nation were saved by said-wardens. A little gratitude is not a hard feat to accomplish.
3. There's a difference between kissing ass and showing respect. Teagan acts like the Inquisition has caused more problems than Orlais...and think about it...it's self-contradicting to be sitting on the same council as the empire that invaded and oppressed your country while condemning and nitpicking the organization that saved your country. Again, it's one thing to be cautious and concerned and other entirely to act as irrationally hateful and disrespectful as Teagan is and it's not exactly consistent with what we've seen of him before. Teagan seems to have shown more respect to Loghain than the Inquisition and that's saying something considering the two situations.
Why should the Inquisition disband? They're not beholden to Ferelden and frankly, Ferelden should've done something about the bandits at Caer Bronach if they didn't want anyone taking the fortress. As Ferelden wasn't even using it beforehand after regaining independence if it can be so easily taken by bandits, they shouldn't be crying over something that they didn't have if it's not disrupting them or if the castle isn't actively harassing their people.
Also, the Grey Wardens were given their own arling in Amaranthine and where's Teagan's outrage there? Oh wait, Teagan didn't take a level in jerkass yet and evidently waited to see what would happen and apparently, that was a good call.
Bottom line, the next time that Teagan gets booted out of his own home, don't expect anyone to help you out if you won't repay good deeds with proper respect and a clear mind rather than blasting both of the organizations that saved your life.