Is it okay to send a plain text message?
Yes, it would be fine to send out a campaign in plain text format, as this is still used on a regular basis by marketers. The only limitations of this format are that you can't track open rates and clicks (this can only be done with HTML messages) and you can't format the message.
HTML messages are more likely to get caught in bulk mail filters, so text-only messages may allow you to get better deliverability. HTML messages are accepted by over 80% of recipients and can be made to look more visually appealing, so your industry and audience would be important to consider before you send text-only emails.