In short: No, Yiddish is not a creole.
A creole is a stable language developed from the mixing of parent languages. A creole develops if (and, AFAIK, only if) its speakers were children who grew up speaking what used to be a pidgin as their first language.
A pidgin is a simplified language that develops as a means of communication between groups that do not have a language in common. A pidgin is not the native speech of any entire community but is an acquired language.
Pidgins develop rather haphazardly out of necessity when multiple language groups, for whatever reason, need to communicate with each other on a very regular basis. Bits of vocabulary from each language are put into a melting pot, so to speak, and an ad-hoc rudimentary grammar develops. People are trying very hard to make themselves understood, and the result of that generally resembles a sort of tarzan-speak.
Pidgin languages are not stable and continues to develop in a rather impromptu manner. There is not necessarily any widely agreed upen grammar and will vary widely from speaker to speaker. Certain conventions will inevitably arrive, that is, the features of the pidgin that are used enough will become standard.
Wikipedia lists some notable characteristcs of pidgins:
- Uncomplicated clausal structure (e.g., no embedded clauses, etc.)
- Reduction or elimination of syllable codas
- Reduction of consonant clusters or breaking them with epenthesis
- Basic vowels, such as [a, e, i, o, u]
- No tones, such as those found in West African and Asian languages
- Use of separate words to indicate tense, usually preceding the verb
- Use of reduplication to represent plurals, superlatives, and other
parts of speech that represent the concept being increased
- A lack of morphophonemic variation
Note that a creole is linguistically more developed than a pidgin. The language is mostly stable. Unlike pidgins, which along with moribund languages are notable for their simplified characteristics, there are no grammatical features that are unique to creoles.
Now, finally, to your question: Yiddish was not a language that developed from two language groups trying to communicate with each other. It is a High German language that was (and, to a lesser extent, still is) spoken by Ashkenazic communities in central Europe. Yiddish was never a rudimentary mixture of two languages, it was just German that borrowed a few features of Aramaic and Hebrew.
And, no, English was not a creole during the Norman conquest. English was still English, it was, for the most part grammatically the same as it was before. English just borrowed a large amount of vocabulary from French.