The Covetous curse states (emphasis mine)
You must wear fine non-magical clothing and jewelry worth at least 50 gp + 100 gp per character level you have beyond 1st. If you do not have sufficient wealth to purchase this additional equipment, you feel a strong desire (but are not compelled) to sell existing items or steal from others to obtain it. You are sickened whenever you do not meet this requirement; you are also sickened for 24 hours after anything worth 25 gp × your character level or more is taken from you against your will.
while the Plagued curse states (emphasis mine)
You take a –1 penalty on all saving throws against disease or infestation effects, but you are immune to the sickened condition.
If I'm not mistaken, then RAW that means you don't get any drawback from the Covetous curse, even when not wearing fancy clothing. But since the second curse isn't supposed to give you benefits only, that does seem kind of wrong.
Is there any official ruling on that? How would you handle that?
Right now, I would probably go with "The character is immune to the sickened condition from all sources but the Covetous curse" or have the player play up the mentioned desire they have (when not wearing fancy clothing) to sell or steal items to make roleplaying encounters more challenging in return.