There is an option to enable anti-aliasing in font settings.
Check Anti-alias under Font in object properties.
However, Rainmeter uses D2D to do rendering, whereas, XWidget still uses GDI+ (See for an explanation). D2D has an added convenience of device-independent pixels, the and in RM, as I've noticed, the base resolution is somewhat 72 dpi, as opposed to XWidgets' 96 dpi. So, a 10px font will match to something about 12 px in XWidget. So the Anti-aliasing done here can never match that of RM.
This is just a reference, I haven't been to RM, for years and have to scuffle with the docs to give you a measure.
Good Luck, looking foreword to see your widgets!
Since, texts are vectors, It should not be much difference, let me dig out what XWidget does behind-the-scenes... and if someting is missing, I put a feature request