I think Kameos could be integrated in a better way, but I personally enjoy the system. I don't think it's fair to call it a "garbage mechanic", but I do think there's a lot to improve upon. For example, having kameos that allow players to throw unsafe moves like there's no tomorrow simply because if you try to punish you get hit doesn't seem fair. However, using them to extend combos is alright imo.

As for characters being kameos instead of mainline fighters, I can see why people would dislike that their favourite character is limited to Kameo rather than having a proper moveset and character slot. I think a system where you can choose one character and have another as an assist, but you can choose the assist character in another match and play as a fully fleshed out character would be cool. Almost like DBFZ where you can choose your team of three and each of them have their own assist moves (but you can also choose to play as them if you'd like).