A person who thinks nasty things could be described as pessimistic, cynical, or having a negative mindset.
A person who buys a lot of things is often described as a shopaholic.
An optimist.
stupid, loser
A person who ignores important things could be described as apathetic, negligent, or careless.
Someone who constantly thinks and reflects on various topics can be described as contemplative or introspective.
A person who thinks highly of themselves is often termed as egocentric or narcissistic.
Atheist.
A cosmologist
Ignorant.
zen master
Someone who doesn't have a life. If you meet someone like this tell them that they need to be more social.
Juvia refers to herself in the third person. For example she says things like "Juvia thinks that" when talking about herself.
God
A person who thinks only of themselves is often referred to as selfish or self-centered.
A person who thinks big and has ambitious ideas is often referred to as a visionary or a dreamer. They may also be described as having an expansive or innovative mindset.
If a person thinks they are funny when they are really not, I would call that person a failed comedian, and possibly a fool. Or maybe the person lacks self confidence and that's the only way he know to bring attention to himself.
An opinion