Much more+ time-consuming
Much more+ annoying
Much more+ selective
Much more + jovial
Much more+ responsible
It basically just depends on the suffix of the adjective and the number of syllables. There are certain endings that sound weird when attached to -er, such as the examples above.
You wouldn’t say jovialer, selectiver, responsibler, time-consuminger, annoyinger, or dangerouser. You’d just initiate these expressions with “more”. The main reason why you’d use “more” is because these adjectives have more than one syllable and the suffix is not “y”.
Words that don’t need “more” are nice (nicer), pretty (prettier), easy (easier), big(bigger), short (shorter)... The main reason why you’d use -er is because these adjectives have one syllable.