Ignoring any early-distribution penalty, a rollover to a Roth IRA is subject to ordinary income tax to the same extent that it would be subject to ordinary income tax if the distribution was paid to you instead. Tax withholding can be avoided by doing a direct rollover to the Roth IRA (which is probably the reason you saw no tax withholding for doing the rollover). If an in-service (not hardship distribution) is permitted, the plan is required to allow a direct rollover to a Roth IRA. If the distribution is instead paid to you and you roll the distribution over to the Roth IRA yourself within 60 days, a minimum of 20% withholding is required by federal law (and there might be mandatory state withholding as well, depends on the state), so you would have to use other funds to complete the rollover to the Roth IRA of the portion withheld for taxes, then get the tax withholding back as part of your tax refund.
The assumption here is that you are asking in regard to 2018 since it's to late to treat any distribution from the 401(k) as income for 2017.
