If "thoughtful" means "considerate" or "kind":
* Thoughtless - lacking consideration for others.
* Inconsiderate - not thinking of the needs or feelings of others.
* Selfish - concerned only with one's own needs and interests.
* Rude - impolite and disrespectful.
If "thoughtful" means "reflective" or "pondering":
* Impulsive - acting without thinking.
* Careless - not paying attention to detail.
* Superficial - concerned only with surface appearances.
* Unreflective - not thinking deeply or seriously about something.
If "thoughtful" means "having or showing careful thought":
* Careless - not paying attention to detail.
* Neglectful - failing to give proper care or attention.
* Inattentive - not paying attention.
* Rash - acting without thinking carefully.
The best opposite for "thoughtful" will depend on the specific situation and what you want to emphasize.