Yes Mucinex is good for a sinus infection.
Any medicine that contains acetaminophen, phenylephrine and guaifenesin is good for relieving a sinus infection and muninex contains those ingredients.
Most sinus infections go away on their own without treatment but taking some over the counter medicine can help ease the symptoms of the sinus infection and help it go away much sooner.
Use a humidifier in your room as well which helps keep your nostrils moisturized.
The humidity can help you breathe much better as well and even taking steamy shower can help.
For some sinus infections antibiotics can help but for some sinus infections there's not much help you can do to get rid of it any sooner.
I have taken mucinex for a sinus infection and it helped ease the symptoms and helped it go away about 4 days sooner.