Tertiary carbanion in more alkylated alkene
There are two types of description of hyperconjugation: (1) Valence bond approach &amp; (2) Molecular orbital approach. Your question is pertaining to valence bond approach as you are interested in the stability of the canonical forms.

In Valence bond approach, hyperconjugation is said to happen due to conjugation of C—H bond electron pair with adjacent double bond. We call this phenomenon 'hyper'-conjugation because there is no real negative charge that is getting delocalized. Rather, the partial negative charge which is accumulated on the carbon due to electronegativity difference with hydrogen is getting delocalized.

In a real carbanion, the carbon has ~100% share of the negative charge. However, in this case, the carbon has only partial share of the negative charge. To be precise, about 7%, because 7% is the ionic character of a C—H bond (Calculated using Hannay-Smith's Relationship for Calculating Percentage Ionic Character [1]).

In case of conjugation of real carbanion, the ~100% negative charge is getting delocalized, and you can give importance to the stability of canonical forms to detrmine whether that conjugation is relevant. However, when ~7% negative charge is getting delocalized, it is more important to give importance to the number of such conjugations, than the relevance of each conjugation based on stability of canonical forms, because anyway, ~7% negative charge won't cause much destabilization even in a tertiary carbanion!

In Molecular orbital approach, hyperconjugation is said to happen due to overlap of bonding σ oribitals of C—H bond with adjacent antibonding π* orbital (in case of double bond). Owing to energy &amp; orientation mismatch, the overlap is minimal and thus stabilization arising from hyperconjugation becomes significant only when the number of hyperconjugations is greater. So, this explains why 2,3-dimethyl-but-2-ene assumes much greater stabilization from hyperconjugation compared to 2-methyl-propene.