The thing about British accents is, in reality there's a lot of them. That's
a British accent, and it's actually not that bad. Really! Put it this way, I didn't even register it as something noteworthy when I watched the trailer. But I've spent time with people who do sound like that. Anyone who hasn't may genuinely think it's over-the-top, but there really are people who sound like that.
https://www.techradar.com/uk/news/a-defense-of-moon-knights-accent-by-an-actual-cockney
Of course, having said that, Marc Spector is actually American so technically I guess the accent
is being put on in the context of the show anyway.