The issue is that the act of gluing itself to the top of the Mech would probably increase the amount of force its body did take ten fold. If it was not stuck to the top it would probably just have been shunted straight off the front of the Mech, and hurt its ass, however by resisting falling off the Mech its ass had to instead essentially take the 30 tonnes of the Mech's body slamming into the wall. It's hard to describe unfortunately, but I suppose you just have to imagine it like a giant block of wood with a smaller block of wood on top overhanging, it you threw the two blocks at a wall the upper piece would shoot the opposite direction upon hitting the wall, but if it were glued down it would take the full force of the bottom blocks weight and prevent it making contact with the wall until either it broke or the glue broke. I suppose in a sense Slimy did break, but not perhaps as much as would be necessary considering how fat Higan's mech is.

Still, some damage was taken and it -pretty- much makes sense, so no complaints. We'll see what happens next in the wonderful world of Mechs vs Alien vampires.