Is my guinea pig's head tilt after Revolution for lice a side effect?

Try not to panic if Sasha has a mild tilt, it could simply be coincidental. The most common used topical treatment is ivermectin, and this is what we know most about, however, there is much resistance to this so quite commonly vets are prescribing selemectin instead. This is the ingredient in revolution and is from the same family as ivermectin. It would seem that it is safe to use, it is very difficult to go wrong with a spot on unless a big overdose is given. You could double check the dose with the vet, but as long as a dose was worked out based on her weight or an average weight for her size then it should be completely fine. If the head tilt is deteriorating or not improving within a few days I would take her to the vet asap. If she has lice it is possible she could have parasites in her ears also that could be the cause.
Best of luck.