I liked MU and TGD (Brave was okay and Cars 2 was just a mess) but like you said, they're not the greatest Pixar films of all time — and Finding Nemo is one of the greatest Pixar films of all time. So if there's a sequel and it's not as good as the original, then disappointment will be inevitable - especially if it's a sequel. MU was a prequel, whereas Finding Dory is a direct continuation of the story, much like Cars 2, Toy Story 2, and Toy Story 3 — and although having an MU-level (in terms of quality) film in Finding Dory would be passable, I'd much rather have a TS2 or TS3 level film.
Cars 2, Brave, MU and TGD are all (with the exception of maybe Cars 2, the only one of these four that's actually Rotten on Rotten Tomatoes) passable, but what sets Pixar apart from the competition is that they're the best of the best, and if they're just average, then that negatively affects their reputation - because we've come to expect Pixar to offer us something their competitors can't. They're the one animation studio that always (with the exception of the previously mentioned four films) goes the extra mile, that's always within the realm of perfection - they've set a very high bar for themselves (seriously, find one person who legitimately doesn't like Finding Nemo) and if they can't reach their own bar, then that's not the Pixar we associate the Pixar brand with.
Although there's definitely plenty of people (like you) who legitimately like Brave, MU, TGD, and even Cars 2 (personally, I think TGD is very close to Finding Nemo in terms of quality - even though Nemo is overwhelmingly considered, generally, the far, far more superior film), you have to keep in mind that Pixar is going after the masses - each Pixar film is a huge blockbuster that tops the box office, gets all the awards, become instant classics, etc. They're not indie/art house films that play to a niche audience at film festivals and later become cult favorites. Therefore in the case of Pixar, the opinion of the majority is very much relevant, and can have significant impacts. Cars 2 was the first Pixar film to not go to the Oscars, putting an end to the awards streak that later not only set a precedent for MU and TGD, but made people hesitant to support even Inside Out like they did for WALL-E/Up/TS3 - a sticky situation I've discussed in further detail in the Inside Out forum. The Good Dinosaur was Pixar's first box office flop, and even though it made tons of money, it wasn't enough to cover the production and marketing costs, which are huge since it's Pixar we're talking about here — and Disney is not gonna be happy about that (funny how Andrew Stanton and Brad Bird both made financial flops with Disney's live action studio, and immediately after they're back at Pixar making sequels that are guaranteed blockbusters regardless of quality).
Finding Dory will probably be a huge money machine regardless of the quality of the film. However, if it's merely MU-level good, its devoted fan base will probably become a niche community, and it won't go down in history as a classic like the original. And since it's the sequel to Finding Nemo, any discussion of quality will immediately generate a direct comparison to the original - at which point Finding Dory is labeled simply as another sequel that failed to live up to (or, in a more extreme case, ruined) the original film. Now compare that to the legacy of Toy Story 2 and 3, both of which are heralded as classics on the same level (or even above) the original film, and are much more beloved by audiences than, say, Cars 2.
So anyway, that's the reason I'm really, really hoping Finding Dory will live up to the original. Expectations are high - we've seen the minds of geniuses at work with Finding Nemo, and with how amazing that film was, wouldn't it be great if the same artists could reach their own bar with round two? Like how they did with Toy Story 2 and Toy Story 3, amazing films that expanded on the original Toy Story in the most wonderful ways, unlike Cars 2 or MU, both of which, although admirable efforts, have obvious flaws that put them a step below the original films in their respective franchises. In conclusion, a MU-level film in Finding Dory is passable - but a Toy Story 2-level film in Finding Dory (or perhaps better?) is what Pixar at the very top of their game would do. And I'd love to see Pixar at the top of their game with their next film