It doesn't fit stylistically Halo, or at least the UNSC. Covies have 4-legged walkers and they're okay, but i'd complain about two-legged ones if they had any. But two-legged mechs... nope. Big mechs are way too unrealistic (they only work in mech-scifi like Battletech), at least the Mantis is small.
The Mantis certainly doesn't fit the UNSC due to how it looks... those thin leg connections and generally slim build, in a faction dominated by bulky things and utilitarianism (well sort of, many UNSC things are not that practical).
EDIT Art of Halo 4 has Mantis concept art. Not a single one would fit the UNSC's style, not the bulky ones, not the flimsy ones. If there were a concept art that fit, i'd say that. The Goliath in Starcraft fits Terrans but the Viking and Thor do not, IMO. I'm not opposed mechs automatically, but they need to be well thought out and fit stylistically to the faction that would have it.
A mech is not scaled-up powered armor. Square-cube law, you see (presumably one could make them workable with anti-gravity tech, but if you have that, why bother with walkers, why not make a hover tank? 4-legged walker with AG assist might work in rugged ground, for non-combat role as the Scarab is meant originally, though it having AG assistance is pure speculation).
And of course, gameplay-wise the Mantis doesn't work well, though it'd call it a tad more balanced than the Scorpion, though this depends on the map of course (in Valhalla, the tank would be utterly OP, in Longbow it is just OP).
I would rather had the UNSC have a hover anti-aircraft tank than the Mantis (which is, fictionally, AA-platform). Or the Halo Wars Cyclops, though presumably a Spartan cannot fit in one.