* If tomatometer & audience score are within 5% of each other, you can trust the ratings to give you a decent indiciation of movie quality.
* If tomatometer is more than 15%+ higher than audience score, it means it's an artsy fartsy movie that critics like and movies don't.
* If audience score is 15%+ higher than tomatometer, it's a fun movie even if it's not oscar worthy. (https://www.rottentomatoes.com/m/old_school is a perfect example)
---
The Last Jedi
Tomatometer 91% Audience 41%: Artsy Fartsy
[Really?]
---
The Greatest Showman
Tomatometer 56% Audience 86%: Fun, not oscar worthy
[Won Oscar for Best Original Song]
---
EDIT: Truthfully, it was the release of these two films (both Dec 2017) that caused the Tomatormeter and I to part ways. Simply indefensible, IMO.
So highest-grossing doesn't mean absolutely anything about how good a movie is, or whether people actually liked it. There is a huge contingent of people that follow religiously a franchise and will pay to watch the new one even if they've been told it is not very good. You don't skip the latest Star Wars movie if you call yourself a Star Wars fan, and it is marketing's job to create the Star Wars fan in the first place.
As a cynic, I'd argue the opposite to what you said: you have to spend more on marketing if the material is not very good in the first place. The result is a terrible movie that becomes a meme and still makes bank.
Marketing can boost ticket sales, but there are plenty of examples of movies which flopped despite heavy marketing, just because the audiences didn't like the movie that much.
The ratings on RT and IMDB does not represent the average audience member.