It doesn't exactly even out, no, because SMP is treated as earnings and OP is likely to be eligible for the work allowance which means some of your earnings are ignored before they make deductions from your UC.
For example, a single mum's maximum UC could be:
standard allowance (single 25+) 393.45
child element (1 child) 287.92
total 681.37
(not including housing element if renting)
In a month when her income is zero (no SMP paid) her UC is £681.37.
One month's SMP is £797.46 but in a month when she is paid it, her UC won't be reduced by that much. If she is claiming the housing element her work allowance is £404 which means that her UC will be reduced by 55% of her earnings above that amount. So £216.40. This means her UC will be £464.97.
If she got two SMP payments in one month, that would be £1594.92, and her UC would be reduced by £655 so she would only get £26.37 that month.
To recap:
Earnings zero UC £681.37
Earnings £797.47 UC £464.97
Earnings £1594.92 UC £26.37
With zero pay one month and double pay the next month, her total UC is £707.74.
With normal pay both months, her total UC is £929.94.
So she has effectively lost out on £222.20.