Stack Exchange generally sends all emails in both HTML and plain text format. For example, here's a segment from a raw email that they recently sent me:
--76fa4222a6c9d0420e262faec774d52c6777ade73006da5b1f792ad1ba12
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset=UTF-8
Mime-Version: 1.0
1 new item in your Stack Exchange inbox
# Your bounty on question "Hide Hot Questions?" expires in on=
e day.
### bounty expires in one day | Oct 27 at 0:00
=20
=20
---------------------------------------------
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--76fa4222a6c9d0420e262f2ec774d52c6777a6e73001da5b1f792ad1ba12
Content-Transfer-Encoding: quoted-printable
Content-Type: text/html; charset=UTF-8
Mime-Version: 1.0
<!DOCTYPE html>
<html lang=3D"en" xmlns=3D"http://www.w3.org/1999/xhtml" xmlns:o=3D"urn:sch=
emas-microsoft-com:office:office" xmlns:v=3D"urn:schemas-microsoft-com:vml"=
>
You can see that it includes a simplified plain text version of the email before the HTML version.
There is no option to change the format Stack Exchange sends you, but your email client may have an option to display the plain text version instead.