It's very likely related to how it doesn't look to be a serious project, especially with its name being Grin and the logo being a lousy emoticon.
Another aspect people dislike about it is its inflation rate, which is 60 GRIN per minute and it will be like this for ever as long as this rule doesn't get forked out. It's admittedly a lot, but the good thing is that it will allow the coin to have a sustainable mining ecosystem with consistent block rewards, unlike Bitcoin where the fees will eventually have to pay for the security.
Do I think it's a shitcoin? I'm on the fence about it, but I can see why people do consider it a shitcoin.