Do isomers have weak covalent bonds as compared to normal hydrocarbons (alkanes)?
You'll have to qualify your question. Isomerism does not imply a weak covalent bond.
Isomers are species of the same chemical formula, but DIFFERENT connectivity. The subject is usually broached in organic chemistry, because carbon chemistry offers great opportunity for both
In both of the given isomers,
Geometric isomerism is a further refinement of isomerism. Geometric isomers have of course the same formula (because they are isomers!), and the same CONNECTIVITY but different GEOMETRY. You will have to seek illustrations of examples in your text.