You can try that.
I normally use openssl-based libustream, but just installed mbedtls for testing for you.
You do not need to uninstall libmbedtls, as it should be enough to install libopenssl, uninstall libustream-mbedtls and install libustream-openssl.
I went to the other direction for testing, so you should do the mirror thing than I...
opkg update
opkg install libmbedtls
opkg remove --force-depends libustream-openssl
opkg install libustream-mbedtls
rm -rf /tmp/luci*
/etc/init.d/uhttpd restart