Will a Canon EOS 500D with a Tamron 90mm macro give good macro image quality?

From your comments:

the colours aren't as bright

Colours from cameras are almost entirely down to how the image is processed. The default settings for compact cameras and (particularly) mobile phones tend to have higher contrast and saturation than SLRs, so photos from SLRs can look a bit flat if you're comparing them with those from a compact camera. But there's nothing stopping you increasing the contrast and saturation, either in-camera for the JPEG output from the camera, or in whatever tool you're using to process the images.

The Canon shots have never seemed very sharp

While it's true that sharpness is limited by the sensor and the lens at the top end, it's much more limited by the technique of the photographer. As has been commented, the depth of field from an SLR is much smaller than that from a compact camera, so you need to be much more careful with your focusing if you want to get the sharpness on the objects you want it on. It's very worth reading why are my photos not crisp? for a lot of things that you can try to improve. Don't buy any new kit until you're sure that it's your kit that's limiting you rather than your technique - particularly macro kit where your DoF becomes razor-thin.