Dr. Ivo Robotnik vs Mortamor: who would win?

Mortamor favored — Slight edge (~55%)

Dr. Ivo Robotnik · Sonic · Base

Tier
2-C, up to 1-C
Attack Potency
Low Multiverse Level, up to Complex Multiverse Level
Speed
Infinite, up to Immeasurable
Durability
Low Multiverse Level, up to Complex Multiverse Level

Mortamor · Dragon Quest · Base

Tier
2-C
Attack Potency
Low Multiverse Level
Speed
Immeasurable
Durability
Low Multiverse Level

A heuristic estimate from normalized VS Battles Wiki stats: tier, attack potency, speed and durability.