I was walking in tall grass at Route 225. We usually encounter Fearow and Raticate. I encountered both of them but when I started walking again what appeared is a Spearow and Rattata. Next, it became normal. I encountered Fearow. But the level is Level 20. It isn't supposed to be Level 20. Is there anything wrong in Route 225 or is it normal?
See the Route 225 guide - all those Pokemon appear.
If you mean a Fearow appear at level 20 then no that shouldn't happen. It was probably the Spearow.