I want to indicate that one approach to solve a problem is less preferred than another approach.
How to phrase it correctly?
Is it grammatically correct to state it this way?
Modeling x and y is a less preferred approach.
It is grammatically correct. I would say "X is a less preferable approach than Y", but "X is a less preferred approach than Y" is valid, too. The former indicates that the preference is based on objective standards, while the latter implies that there is a specific person or group that actually does the preferring.