You are opening the can of worms regarding text.
Degree symbol is NOT standard ASCII. Hence, beauty is in the eye of the beholder (or the rendering is based on the code-page of the system receiving the text-based email).
There are articles upon articles about this issue, starting with how you enter it (does/how does your editor support non-standard ASCII), and then how it's rendered (how the recipient perceives it).
Since Designer only supports basic text email (not HTML), you are kind of stuck.
You could view it in Data View as Quoted Text, then you will get the $-escape sequence value (probably "$BA"

). Regardless, your recipient needs to see it. If it's more than 1 recipient, you might have to send a different email for each recipient depending on how their specific email client views non-standard ASCII text emails.
This article
https://stackoverflow.com/questions/67536302/plain-text-email-not-able-to-display-non-ascii-characters says a good email client may be able to comprehend a plain text email with extended ASCII as UTF-8 (one version of Unicode encoding). The UTF-8 Unicode characters for the degree symbol are 0xC2 0xB0, so "$C2$B0" in Designer. It won't display correctly in Designer (it uses ANSI, not Unicode), but the email recipient may see it correctly.
Or, just spell out "degrees"

.