You can install air conditioner mounting brackets yourself if you have the necessary skills, tools, and confidence in handling such tasks. DIY installation can save money and provide a sense of accomplishment. However, it requires careful attention to detail, as improper installation can lead to safety hazards, such as the unit falling or causing structural damage. 
Before proceeding, ensure you have the right tools, such as a drill, level, measuring tape, and appropriate screws or bolts. Follow the manufacturer's instructions closely, and make sure the brackets are securely attached to a sturdy part of the wall or window frame. It's crucial to check that the brackets are level to ensure the air conditioner operates efficiently and safely.
On the other hand, hiring a professional is advisable if you lack experience or the necessary tools. Professionals have the expertise to ensure the brackets are installed correctly and safely, reducing the risk of damage or injury. They can also provide advice on the best placement for optimal cooling efficiency and may offer a warranty on their work.
Consider factors such as the complexity of the installation, the weight and size of the air conditioner, and your comfort level with DIY projects. If the unit is particularly heavy or the installation site is challenging, professional installation is recommended. Additionally, some warranties may require professional installation to remain valid.
In summary, while DIY installation is possible, hiring a professional ensures safety, compliance with warranty requirements, and peace of mind.