for clarity: those are Homestuck references, because Marigold is wearing a Homestuck shirt
specifically, the shirt references this charming young fellow)
Actually, that's the zodiac symbol for Cancer, the crab. Both my brother, and ironically my daughter - the one who was struck by cancer - are July babies.
Since the homestuck guy is also clearly cancer-related, it makes sense, but ... it's more general than that!
Yes, that is the Cancer symbol, but no, it's not more general.
Or at least I really, really doubt it's more general. Marbear is an obsessive fan, and Homestuck collects obsessive fans like iron filings to a huge magnet. Also, the fact that it's grey on black is a giveaway (namely that fans can actually buy a shirt that looks like that).
I also recall Jeph saying on Tumblr that he thought well of Andrew Hussie, the author of said multimedia clusterfuck, but couldn't get into Homestuck himself...so I think this is a quiet tribute.
..... yeah, I'm a huge fucking nerd, fight me ._.