Recently it has been seen that some countries have accepted Bitcoin as legal tender. But my question is more adoption of btc and taking it as legal tender aren't going to harmful for the economy of a country, specially when btc is in bearish ?
This is imho quite a good question, actually.
While some will say it doesn't matter it's a bear market since Bitcoin can easily be used only as means of payment, ... it's more complicated than that. Bitcoin price can fluctuate a lot.. in both directions. If one gets his salary in Bitcoin and in a matter of days his salary's value drops by 10%, he won't be happy.
That's why I've found it interesting the way El Salvador did; one can still use internally fiat (USD) if he wants to, while he does accept Bitcoin. If he decides to keep/invest into Bitcoin, it's on him and, depending on his patience, he may or may not earn big.
Someday, not too far in the future, the price should no longer have wild fluctuations and then it will be more suitable for adopting bitcoin as legal tender.
Or is it better to use it through control to avoid illegal or the bad side effect on economy?
I don't understand this. Adopted or not, they still cannot control it. I don't think it has too much effect even on El Salvador's economy, although they have indeed bought some and the price is down now (they still have USD too as legal tender); on the other hand the tourism there has grown over the expectations.
Many countries have still outlawed it for not being able to control it.
Some are afraid of it, some have their own agenda (CBDCs), some simply want to protect their crappy fiat currency....