Mysterio isn't exactly one of Spider-Man's most lethal foes. Without the kind of powers or savagery that make Green Goblin, Doctor Octopus or Carnage automatically serious threats, Quentin Beck is a decidedly second-tier Spider-Man villain.
Even though he's been fighting the web-slinger since the '60s and is a founding member of the Sinister Six, Mysterio usually uses his illusions as a bit player in larger schemes. With plans that have involved making Spider-Man think he's only six inches tall or stealing Aunt May's newly-discovered fortune, it would be easy to consider him to be something of a pushover. In his plans for Spider-Man 4, director Sam Raimi may have even used Mysterio as a comic relief villain.
However, there's a reason Mysterio is the main villain in Spider-Man: Far From Home. Through his illusions, Beck has proven himself to be an especially effective villain as a master manipulator. At his best, Mysterio is a psychological threat, not unlike the Batman villain Scarecrow. While he might have competent combat skills, Beck's true talent lies in his ability to make his enemies believe things that aren't real and make faulty decisions based on that information.
Even though he's spent most of his criminal career fighting Spider-Man, Mysterio has been a far more effective villain when he's turned his attention to other Marvel characters. For instance, Mysterio played a key role in the revitalization of another major Marvel hero, Daredevil.
In Kevin Smith and Joe Quesada's blockbuster Daredevil storyline "Guardian Devil," Mysterio effectively tore Matt Murdock's life apart. After being diagnosed with an inoperable brain tumor, Mysterio wanted to go out after setting off one grand final plan. Since he was under the impression that Spider-Man was still a clone who he didn't have any history with, Mysterio targeted Daredevil, who had broken up one of Beck's scams a few months earlier.
Disguised as a man named Nicholas Macabes, Mysterio tortured Daredevil by leaving an infant in his care who may have been destined to bring about the end of the world and drugged him to make him more hostile and aggressive. Mysterio tricked Matt Murdock's legal partner, Foggy Nelson, into killing his mistress by making her appear to be a monster.
In the storyline's most controversial threat, Mysterio also tricked Karen Page, Mudock's longtime girlfriend, into believing that she was HIV+ before orchestrating the events that led to her death at the hands of Bullseye.
Even though that story ended with Mysterio admitting that his plan wasn't totally original and killed himself, Page has stayed dead for 20 years. Considering how common resurrections are in the Marvel Universe and how prominent Deborah Ann Woll's Page was in Marvel's Netflix shows, that's quite an accomplishment for Mysterio.
After Mysterio was mysteriously revived by the demon Kindred, he was portrayed as a slightly more competent villain. He was one of the only Marvel Universe characters who had access to the Ultimate Marvel Universe, and he used that access to kill that world's Kingpin and cause trouble there, including one scheme that saw the first meeting between Peter Parker and Miles Morales.
However, Mysterio's darkest, most surprising accomplishment came in the alternate future of Old Man Logan.
In that Mark Millar and Steve McNiven Wolverine story, Marvel's villains team up to make a concerted effort to wipe out the superheroes. Mysterio plays a central role in by making Wolverine think that the Xavier School for Gifted Youngsters is under siege by a cabal of villains. After Logan brutally kills all of them, Mysterio revealed that he had used illusions to make the X-Men look like villains and tricked Wolverine into killing his closest friends.
Thanks to his mutant powers, Wolverine has some of the better senses in the Marvel Universe. Even though he's usually able to detect shape-shifters by smell alone, Mysterio was able to create dozens of perfect illusions simultaneously. While tricking Logan's senses is an impressive feat in and of itself, Mysterio's actions took the X-Men out of action and paved the way for the villains to take over America in that timeline.
While Mysterio hasn't done anything quite that impressive in the main Marvel Universe, he's still causing trouble for Spider-Man. Given what he's already done and his truly terrifying potential, the web-slinger certainly can't write Mysterio off as a second-rate villain.