In my opinion, my issues with USF's aesthetic stylings come down to three things.
A lot of USF has a degree of purposeful "Faux"-ness to it. The 2D-printed character art around Springfield, Minion Mayhem and Dreamworks Land. The temporary convention look of the Villain-Con soundstage. The Mummy queue has you enter through bare, gray soundstage work areas before you enter the highly themed "movie set" queue.
Some of the park doesn't even bother hiding their bare components behind faux theming. Fear Factor Live is explicitly just an ugly gray ampitheatre. Rip Ride Rock-It is a large, ugly rollercoaster station, and nothing more. Neither Minionland ride actually ditched the plain, tan box-building theming from the classic Studios days, despite never actually using that Hollywood soundstage location within the story of the attractions themselves.
Some of these elements are done better than others, but IMO I just can't see most of it without thinking "That was probably the cheapest option they could've chosen". You can make a park look like Springfield, Far Far Away, or even old Hollywood while still keeping it premium and engaging visually.
I actually think E.T. gave the best case for how to upgrade this kind of faux aesthetic, and that's just by making the area around it look really inarguably solid. Give it a nice big expensive sign to replace the older chintzier one, plant some huge Pine Trees and other authentic greenery all around the area, paint some nice mural art with lots of shading and pleasing colors to tuck away the bathrooms next door, and generally hide the tan, boxy soundstage elements just enough to where you don't even notice them before you're transported to a 1980s Californian forest searching for E.T. with Botanicus.
A lot of it just feels actually old and outdated. HRRR is incredibly 2000s-y. The architecture around Animal Actors, BTTF/Springfield, and elements of the park entrance feel very 90s. Lots of repurposed buildings, or simulators that have clearly gone through a few different IP already.
There is some truth to the idea that some dated elements can add charm to a place, and a lot of that can be personal and change with the biases from person to person. The Transportation & Ticket Center or Contemporary at WDW are incredibly 70s-feeling to me, but I am largely okay with that and I think it adds a bit to their charm and feel.
I don't personally see too much charm at USF however. I just see areas that haven't been renovated in multiple decades. It doesn't feel like these dated elements have been preserved out of a love for a bygone era. It feels to me like this park has just been neglected, and they haven't gotten around to replacing these things yet.
Even the elements that are well-themed, or at the very least highly themed, are often placed together in ways that do nothing to aid the general look and aesthetic of the park.
Why is there a giant, cubic military base right along the water between New York and Hollywood? It's a huge imposing monument next to the Minion shops in front of it. It doesn't fit into any land or identifable area of the park. It doesn't even resemble anything specific from real life or a movie. It's designed that way because they needed to quickly retheme their tan box into something less tacky, and adding some steel framing and gray military paint was the easiest, cheapest way to do it. It just looks bad.
The park is better about this then it once was. It used to be Minions, Monsters, Shrek, Music Plaza, and Transformer all within eye-sight of eachother. A truly nonsensical skyline, even within the context of a movie lot. Now, at least we can cross off Monsters and Shrek from that list, and we have added a lot more cohesion between the Minion attractions; but it's still not very appealing to look at.
This park has always been built without a thought to the long-term. Whether certain rides should be preserved as classics, or whether an addition might end up messing with the cohesion or aesthetics of the park around it. It's a result of UOR's early mentality that it needed the biggest, coolest thing to open up as soon as possible, or it would die. Whatever that cost them long-term, they had to build it, or the park would die. It was honestly the truth for awhile, but when Potter struck gold for them, and the other two parks took a lot of that expansion load off of USF, they have been given the opportunity to slow down at the main park. That also means looking back at this improvised mess of singular concepts that now make up the lineup of their flagship park. No cohesion or flow. All simulators, no water rides, very few dark rides, and very little consistent theming.
That hodge-podge structure the park has built for itself means that only a few areas of the park look truly great. I don't think New York or Hollywood are beautiful or ground-breaking, but they look solid for what they're doing. I think Central Park is rather nice, Sting Alley is really cool, and Diagon is clearly one of the best things ever built in a theme park. But most of these are only as good as they are because they are secluded from everything around them.
Looking at the park from the front gates and seeing bright green from Wicked, the Minions clambering on their sign, Jimmy Fallon in the distance; walking further and adding to the tonal and visual confusion with Rock-it and Transformers; it creates an impression that there has never been a singular visual idea for this park. Everything put there was done so because it was convenient, and nothing more. It's an incredibly busy mix of "bad ideas, bad execution" and "solid execution, terrible location" that leads to a finished product that screams
cheap,
dated, and
improvised.