I guess that people can be split in two ways - those that like Kevin Smith's films and those that don't. From watching his films, he seems to like his characters to exist in his strange world where things are exaggerated and ridiculous characters do unrealistic things.
That sums up Mallrats - it's the story of two friends who both lose their girlfriends and then spend the rest of the day hanging around in the local mall. Whilst hanging around they meet friends and get into scrapes as they strive to get their girlfriends back.
I suppose if you looked at it coolly it's all a bit silly - fully of ridiculous situations and scrapes that are resolved in unbelievable ways. But then if you accept Smith's world of comic book style adventures and cartoon film making then this is great. Whereas his later Chasing Amy brings adult subjects into the comedy - this is pure cartoon comedy, although understand it's not dumb like slapstick - but crazy, clever humour with plenty of jokes occuring all around the main action.
OK the overall plot is weak at best, but the story is more about the characters and the situations along the journey to the end of the film and here is where Smith wins. He has created crazy characters that are funny and often exaggerated versions of people or of people's reactions to situations (witness the magic-eye poster guy for an example of exaggerated humour).
Lee is fantastic, this is the role he was made for - he reacts in an exaggerated way to everything and really hams it up. I suppose he's a comic-book reading loser but in this world he is funny and in control. He is loud and abusive to others and it's great! Jeremy London is a weak straight man and doesn't really convince.
Jay & Silent Bob are good as always - although for most of the movie they exist in their own little subplot of taking on the mall police. Again their adventures are exaggerated for humour.
If you hated Clerks and Smith's other movies then you'll hate this. However if this world is one that appeals to you then you'll love this movie's reckless abandonment of reality and enjoy the adventures involved in a trip to the mall.