Can you square up edges on uneven plaster?

Its a fairly simple job if your chimney is big enough to blend the plaster in over a foot or so beyond the edge you have created. Failing that you would need the chimney its self skimming entirely. Hard to say exactly without pictures

You should really have this done in a sand and cement render as it is less likely to shrink in the heat.

Skimming can improve the appearance of uneven plaster by providing a smooth and uniform surface, but it may not be the ideal solution for squaring up edges in all cases. Here are some considerations: Depth of Unevenness: Skimming is effective for addressing minor imperfections and providing a consistent finish. However, if the unevenness is significant or if there are deep gaps, additional steps may be needed before skimming. Structural Issues: If the unevenness is a result of structural issues, simply skimming the surface may not be sufficient. It's important to address the underlying cause, whether it's related to the brickwork, framing, or other structural elements. Square Edges: Skimming can create a smooth surface, but it may not inherently square up edges. For achieving squared edges, additional measures such as using corner beads or manually shaping the edges may be necessary before applying the skim coat. Professional Assessment: Before proceeding with any plastering work, especially if structural changes have been made, it's advisable to consult with a professional plasterer or contractor. They can assess the extent of the unevenness, recommend appropriate solutions, and provide insights into achieving squared edges. Preparation: Proper preparation is key. Cleaning the surface, applying bonding agents, and addressing any large gaps or irregularities before skimming will contribute to a more successful outcome. In summary, while skimming can improve the appearance of uneven plaster, achieving squared edges may require additional measures. A professional assessment and consultation will help determine the best approach based on the specific conditions and requirements of your fireplace opening.

You can put stop on up to the edge and feather it back or skim the whole chimney