Is it permissible to seek help from a deceased mother in a poem or in general, and is it considered unlawful to seek help for the feeling of happiness?

If the supplication of the absent is metaphorical, such as in poetry, and not intended as a genuine request, then it is permissible and does not fall under the prohibition. The prohibition applies when the call is genuine and what is asked of the one called upon is something that none but Allah can do. And asking others for help with happiness is permissible if the intention is assistance through legitimate means that are within the capacity of the one being asked. It is advised to turn away from whisperings (waswas) and to beware of excessive questioning.