If your read this sentence it becomes:
Mehr Glück ist mehr Spaß.
So, here the equal sign clearly stands for the verb of the sentence and it is not any kind of punctuation. It is just some kind of abbreviation. And this again means: No capitalization.
Btw: It is not recommended to replace words in written texts by any special characters. So technically this:
Mehr Glück = mehr Spaß.
is wrong. But of course it will be understood, so in some informal types of text it is acceptable. But in those types of text where it is acceptable (like postings in social media) nobody really cares about correct capitalization.