Captain America's The Avengers' suit looked so ridiculous that it even became the butt (literally) of a joke in Avengers: Endgame. However, there might be an underlying reason why he was given such an uncool ensemble in the film.
Steve Rogers' origin story in Captain America: The First Avenger directly led to the 2012 ensemble flick directed by Joss Whedon. Waking up after 70 years of being frozen in ice, Captain America found himself in the 21st century. Before he could even settle fully in his new life, the star-spangled man was put to work in light of Loki's (Tom Hiddleston) attack at the S.H.I.E.L.D. base. He, alongside five other heroes (Iron Man, Thor, Hulk, Black Widow and Hawkeye) were recruited by Nick Fury (Samuel L. Jackson) in preparation for a looming battle against the God of Mischief. But while all of his new comrades sported their best suits and weapons, he was stuck with a bright blue and red lycra onesie, complete with a cowl that made him more like a mascot than a distinguished war veteran.
At that point in Captain America's MCU history, it's not like he hadn't sported a decent suit. His final battle armor in The First Avenger looked cool, with a more subdued color palette and slightly different design. It was appropriate in terms of the movie's time setting. Steve's The Avengers ensemble, on the other hand, was more reminiscent of the one he wore during his propaganda appearances shortly after he was given the super soldier serum. It definitely looked cheesy - but that may very well have been an intentional decision by Whedon.
In The Avengers, it was revealed that Phil Coulson (Clark Gregg) had significant input in Captain America's contemporary suit, and as a Captain America super-fan with a vintage trading card collection, Coulson likely based Steve's outfit on the cheesy propaganda costume that he was most familiar with. Putting Rogers in a suit similar to his war bonds promoting days also reminded fans of the the same uncomfortable feeling he felt while being paraded on stage.
Aside from a reflection of Captain America's inner struggle to fit in, dressing him up in such an outdated-looking suit was a great visual representation of how he's the real outlying hero in The Avengers - even more so than Thor (Chris Hemsworth). At least the God of Thunder had recently had the time to get acclimated to Earth via the first Thor film. Rogers, on the other hand, had been out for seven decades - unaware of how things actually work in this day and age. Nothing emphasized this more than putting him side-by-side with Tony Stark (Robert Downey Jr.), who was usually equipped with high-tech paraphernalia. Their respective battle costumes were representations of their core characteristics - Captain America with his traditionalist views, and Iron Man with his futuristic approach.
Captain America's The Avengers suit is arguably his worst one. It's tacky, outdated and goofy - and it's also an accurate depiction of how it felt to be in his shoes ,surrounded by people who felt like they were one step ahead of him. Whedon leaned on this idea with the now-famous line where Fury mentions flying monkeys, Thor is confused, and Cap responds excitedly, "I understood that reference!" As bad as this suit was, it's an integral part of Captain America's costume evolution that made for a great visual representation of his overall arc in the MCU.