Me imagino que es una persona a la que le dicen "señor" y el responde:
Decirme, llamarme, señor suena petulante
Referirse a mi como "señor" es odioso.

Saludos,
Glo

About all you really need to know is that this term is derogatory. There is probably a hint of irony in it, depending upon the rest of the story it is in reference to. "sucks" normally is synonomous with "bad", "lousy", "terrible", but of course it can also relate to a vacuum of air. "sucks-a-lot" would thus mean "very bad", "very lousy" or "really terrible". This would be an ironic name for a vacuum cleaner -- something that is supposed to literally suck but yet is not a good product for some reason or another.
Someone let me know if I am on target with this.

Well, everybody here seems to be right.

Vulgar Slang. To be disgustingly disagreeable or offensive

There was a rap artist a few years back who called himself Sir Mix-A-Lot. His name was of course a pun on Sir Lancelot.

Given that your phrases comes from a very youth and pop culture oriented context, the inspriration may come from the contemporary world of rap rather than the old world of chivalrous myth.