Sometimes. Certain models of bidet seats are very DIY friendly. If your toilet has a newer shut off valve in good working condition and you have a level of comfortability with basic plumbing tools and techniques, this may be a doable project. As a decision-making aid, here are some of the things that can go wrong/challenges with bidet seat installation:
The shut off valve doesn’t not fully shut off to the toilet.
The shut off valve has a deteriorated rubber washer that breaks into pieces when operated and clogs up the toilet fill valve or new bidet seat when turned back on.
The shut off valve has a non-standard outlet requiring special adapters.
The water line tee that comes with the new bidet is cheaply made and cracks when overtightened or develops a leak after installation.
The water line tee is cross-threaded onto the toilet fill valve thread and damages them requiring a fill valve replacement.
The water supply lines are not connected/tightened properly and develop a leak after installation.
The old toilet seat bolts are heavily corroded and require special tools for removal.
The new bidet seat has a power cord and requires an electrical outlet to be installed near the toilet.