Unlike pure equity, debt funds have some fixed return. However when one invests though mutual funds, the return and risk are lesser than when one invests directly.
SIP is resorted and becomes beneficial in the 'averaging effect' when the market price is fluctuating. On long term, over the entire average price paid by us may be a reasonable one and not affected by the extremes.
However in the case of debt funds, the return depends on the interest rates n the debts or the return rates. As there will not be much difference in the return rates or interest rates on debts, the prices also may not fluctuate much. So the SIP may not work as good as other funds.
In case of debt funds investment, one should see the portfolio deployment by the Mutual funds and what constitutes the deployment. If the average return by the funds is not much higher than the average bank deposit interest, for a small investor, it is better to invest directly with reputed and established companies or banks itself rather than going to mutual funds.