I think when it comes to comic book based movies, there inlies the problem... people generally think Comic book=Super Hero, and Marvel and DC have too strong a hold in that market for any smaller, independent comic super guys to get their chance in there. Sure, we had Kickanus and Scott Pilgrim manage to break through that ground, but there's just SOOOOO much weird and wild stuff that deserves the big screen treatment. A The Tick movie is SCREAMING to be made. Dude needs a come back from that TV show and bad.
Not to mention the fact that comic book movies aren't all super hero movies. Persepolis? That was a comic book movie (and highly HIGHLY underrated for the look alone. I'm kicking myself for never going to actually see in in theaters and waiting for video). Even that Harvey Peakar movie was based in comics. And if there are 3 non-Super Hero type comic movies that need to be made... Usagi Yojimbo, Groo the Wanderer, and a Disney Life and Times of Scrooge McDuck (NOT Ducktales. There should be NO Launchpad or Webby anywhere near there) all need some sort of treatment as far as I'm concerned.
But on that note getting back to the subject... underrated comic book movies? Simple... All the animated Lucky Luke and Asterix movies. At least here... and I don't know about the live action ones. I heard the last Live action Asterix film did gangbusters, but it also won the European equivalent of a Razzie Award. I would say, I highly recommend Asterix in Britain, The 12 tasks of Asterix, and Surprise at Caesar (or something like that) the most. As for Lucky Luke... only saw the first 2, and I recommend Ballad of the Daltons over Daisy Town.
Also on that note... The First and fourth TMNT movies. The second one, I'll deal in overrated movies, and the Third one, while I don't think it deserves some of the chastising it gets, I think a certain amount of the chastising fits. At least Casey was back in there. But the first one showed how to get a comic book movie done right. Yet no one considers it a comic book movie. Cartoon got in the way. Sure, they gave April her cartoon job (news reporter... no way they could make her a scientist), and changed up a few things here and there for the sake of the story.... but they managed to keep the tone of the comics and just blend in a teeny tiny amount of the cartoon series that made it popular... all while being a separate entity itself.
Meanwhile the fourth one REALLY didn't get the respect it deserved, not even from the fan base. I guess too many people wanted a "Let's go out and beat up Shredder for 90 minutes" movie. They took a story line that would have came right out of Tales of TMNT (the side comic) and managed to make a decent floating sequel without turning it into a lame revenge plot, managing to get Karai into the movie without dumbing it down and explaining her to everyone. it almost felt like an episode of the last TMNT series. But with much higher budget. Too bad that it got a PG rating, when we all know PG is basically just a souped up G rating nowadays.