CrowdCube essentially provide a service, which consists of a vehicle through which to invest in companies, hold your investment and eventually sell your shares. From my perspective, it feels fair to pay for that service.
If they said they carried out extensive due dilligence on those companies, I’d expect them to charge more.
(I write this having recently seen my one of my investments through CrowdCube plummet by over 90%, as that company was bought out for only a tenth of what it had previously been valued at! However, I also have made substantial nominal gains with my investments in other companies, though - as they say - you shouldn’t really count your earnings until you’ve actually crystallised them by selling them!)
Having said that, Monzo may or may not have struck a deal with CrowdCube. Either way, if you feel unable to justify those fees, you shouldn’t invest. And if you think CrowdCube are being overly greedy, you could always invest in CrowdCube themselves to profit from that perceived greed. They’re currently raising funds from existing investors before opening up to new investors, and have apparently raised £1M over the first 24 hours (which, amusingly, is 900 times how long it took for Monzo to raise £1M during their first investment round )