What's your opinion on Elon Musk's decisions as CEO of Twitter from a financial POV? Is the name change to X costing them?

In his capacity as CEO of Twitter; the name change does not make a lot of sense. It is an expensive rebranding exercise with questionable value. Twitter holds significant brand recognition and the public may never switch to the new name.

Elon might have a broader strategic plan in the long term that crosses his portfolio but when looking at Twitter in isolation it probably wasn't the wisest decision.

Yes it not a smart marketing move, rebranding of Twitter logo would have been a better choice. World over twitter logo had it own identity and branding. 
From financial POV it will degrade the valuation of this premium brand.

I believe that Elon Musk has got trapped with Twitter due to a wrong decision. The decision was wrongly taken due to inflated number of accounts shown on the portal. The decision and actions are being taken to do damage control, but the decision like name change to Social (X) have a significant cost impact. The decision to acquire and manage Twitter have gone drastically wrong so far.