If there's anywhere it gets a positive rating then it's fake and manipulated. That will always be the response.
To be fair, just like almost any system there are issues with the IMDb rating and they use a secret weighted average adjustment so no one really knows how that rating is made. Their stance is it is there to try to combat artificial reviews and brigades (positive or negative). However, you can still see the raw unadjusted rating.
Also, I have found that older films tend to rank better (in the moment and over time) simply because people are less likely to seek out films they hate to leave a bad score unless it is recent.
For those who all think it's fake, you have to wonder why Supergirl is a 6.1, the Flash is 6.6, and The Marvels is 5.4. Those mega companies just didn't pay enough to rig the system for their mega budget films?
If a movie doesnt connect with the general audiences and flops (I am talking "big name" movies like the three you mentioned), the scores are reflective of people that actually saw it generally. Did a brigade of fake reviewers need to leave bad reviews of Supergirl that they never saw? Some maybe but the people that actually went did too and well not that many people went.
Really all this shows is All Hail The Mighty Cinemascore. The score from people who get polled coming out of the theater if the movie met their expectations and if they would recommend it to someone they know to watch it as well, since well people's ultimate decision on a movie that they were unaware of or unsure of is more generally based on recommendations from friends and family. And generally those people will go after the first week which will indicate the legs a movie will get. I actually got a ballot once for Dark Knight. This got an A with 80% suggesting they would recommend it to someone they know. That strongly indicates long legs and a solid second weekend. Plus it being rated R means parents will go see it later as they need to arrange a babysitter etc.
Does this apply to kids movies? Not overly well, as kids are stupid and like anything and ultimately the adults are the decision makers. Moana live action got a A- but is bombing horribly. Why? Because well its a remake Moana movie that is 10 years old and it was Moana and if you enjoy Moana you would probably like it but adults look at it and go, this shit is on Disney Plus I dont wanna spend $100 for my family of 4 to go see essentially the same movie. Where as Moana 2 which came out less then 2 years ago with the same Cinemascore made over 1 billion (psst new plot actually helps more then live action Rock Disney). Toy Story 5 got an A, has nostalgia built in, and its a new story and its almost at 1 billion.
Supergirl: B-
The Flash: B
Marvels: B (lowest score in MCU history)
B for a super hero movie is a death sentence.Weak presales, middling RT scores, etc can also set off alarm bells... but if it didn't open strong and the people who did go and are most likely to want to like it didn't nor overly recommend it, it cuts the legs out from the film and it flops. Lets be real, people want to be entertained by these flicks, and if Supergirl wasnt some child trafficking pseudo party girl thing that look bad and had a bad script, it probably could have done well. But audiences hated it and noone said to me "Hey go see Supergirl, its actually pretty good", so ill never watch it even though I am rarely swayed by culture war BS.
B for a horror movie is genre specific and generally considered a solid result, could be slow burn and audiences thought it would be Blumhouse jumpscares. Sinners is the ONLY horror film to have an A, general horror audiences loved it and it made a pile of cash, it doesn't mean its the best horror film, just one that connected with its audience.
A D for Mother by Darren Aronofsky makes it super interesting to me, full audience rejection movies are super interesting in their own way. I didn't love it, but it did make me wonder how it ever got a theatrical release lol