As you say, “camelCase” is used more often than “PascalCase” or “snake_case”, although it's more a convention than correct/incorrect question. As long as sender and receiver agree to the used naming convention, it's functionally ok.
Using Mendix message definitions, automatically PascalCase will be introduced, however: if your attributes contain underscores, it may become a mix.
So my recommendations:
See also my idea to make this a global configuration in Mendix Studio Pro: https://forum.mendix.com/link/space/integrations/ideas/3783