If it is a formal business email, it is impolite to send it without typesetting on the mobile phone; If it is an informal email, typesetting itself is not a question of "politeness".
The signature area should be used to introduce yourself, not to write what you want to say to the recipient. I feel that I can't express my respect or apology for the sender by saying sorry in general in my signature, which is completely different from the apology in the text.
So, sending from iPhone is just a device stamp. Apple uses this for advertising. If you don't mind, just keep it. Delete it if you mind. If you want to show your personality, change it. If you want to leave your contact information, career information and company slogan, you can also. However, it is really not used to say sorry.
If everyone uses this signature, people who don't have an iPhone often see it, so they won't mistake it for showing off.